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{{short description|Color}}
{{pp-pc1}}
{{about|the color|other uses|Indigo (disambiguation)}}
{{about|the color|other uses|Indigo (disambiguation)}}
{{redirect|Violet-blue||Violet blue (disambiguation)}}
{{redirect|Violet-blue||Violet blue (disambiguation)}}
{{distinguish|Indego|IndiGo|IndyGo}}
{{Use dmy dates|date=July 2012}}
{{Use dmy dates|date=October 2019}}
{| align="right"
|+ Indigo as a [[tertiary color]] on the RYB color wheel
|-
|{{legend|#0247FE|blue}}
|-
|{{legend|#4123BF|'''indigo'''}}
|-
|{{legend|#5517AA|violet}}
|}
{{infobox color
{{infobox color
|title=Indigo
|title=Indigo
|image=File:Indian indigo dye lump.jpg
|textcolor=White
|image= File:Indian indigo dye lump.jpg
|caption=A piece of indigo plant dye from [[India]],<br />about {{convert|2.5|in|cm|0|abbr=on|order=flip}} square
|caption=A piece of indigo plant dye from [[India]],<br />about {{convert|2.5|in|cm|0|abbr=on|order=flip}} square
|wavelength=450–420<ref name="EoF">{{Cite book|url=https://books.google.com/?id=HQWNJyRV6kMC&pg=PA189&lpg=PA189&dq=%22Indigo%22+++%22450-420%22#v=onepage&q=%22Indigo%22+++%22450-420%22&f=false|title=Encyclopedia of Physics|first=Joe|last=Rosen|date=26 June 2017|publisher=Infobase Publishing|via=Google Books|isbn=9781438110134}}</ref>([[#Classification as a spectral color|disputed]])
|wavelength=420–450<ref name="EoF">{{Cite book|url=https://books.google.com/books?id=HQWNJyRV6kMC&q=%22Indigo%22+++%22450-420%22&pg=PA189|title=Encyclopedia of Physics|first=Joe|last=Rosen|date=26 June 2017|publisher=Infobase Publishing|via=Google Books|isbn=9781438110134}}</ref>
|frequency=~714–670
|hex= 3F00FF
|hex=3800FF
|r= 63|g=0|b=255|sRGB=1
|source=[[CAM16|CIECAM16 248:2022]]<ref name="css3-color">[https://cie.co.at/publications/cie-2016-colour-appearance-model-colour-management-systems-ciecam16 CIECAM16 248:2022] {{Webarchive|url=https://web.archive.org/web/20240420151156/http://cie.co.at/publications/cie-2016-colour-appearance-model-colour-management-systems-ciecam16 |date=20 April 2024 }}. J = 41, C = 74, h = 289</ref>
|c= 75|m=100|y= 0|k= 0
|isccname=Dark violet}}
|h=255|s=100|v=100
|source=[[Web colors#X11 color names|HTML/CSS]]<ref name="css3-color">[http://www.w3.org/TR/css3-color/#svg-color W3C TR CSS3 Color Module, SVG color keywords]. W3C. (May 2003). Retrieved on 14 December 2007.</ref>
}}
'''Indigo''' is a deep and rich color close to the [[Blue (RGB)|color wheel blue]] (a primary color in the [[RGB color space]]), as well as to some variants of [[ultramarine]]. It is traditionally regarded as a color in the [[visible spectrum]], as well as one of the seven colors of the [[rainbow]]: the color between [[violet (color)|violet]] and [[blue]]; however, sources differ as to its actual position in the [[electromagnetic spectrum]].


'''Indigo''' is a term used for a number of [[hue |hues]] in the region of [[Shades of blue#Blue (RGB) (X11 blue)|blue]]. The word comes from the [[indigo dye|ancient dye of the same name]]. The term "indigo" can refer to the color of the dye, various colors of fabric dyed with indigo dye, a spectral color, one of the seven [[Rainbow|colors of the rainbow as described by Newton]], or a region on the [[color wheel]], and can include various shades of blue, [[ultramarine]], and green-blue. Since the web era, the term has also been used for various purple and violet hues identified as "indigo", based on use of the term "indigo" in [[HTML]] web page specifications.
The color indigo is named after the [[indigo dye]] derived from the plant ''[[Indigofera tinctoria]]'' and related species.

The word "indigo" comes from the Latin word {{lang|la|indicum}}, meaning "Indian", as the naturally based dye was originally exported to Europe from [[India]].

The [[Early Modern English]] word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic [[color term|color-naming system]].<ref>{{cite book|last=Ottenheimer|first=Harriet Joseph|title=The anthropology of language: an introduction to linguistic anthropology|year=2009|publisher=Wadsworth|location=Belmont, CA|isbn=978-0-495-50884-7|page=29|edition=2nd}}</ref>


The first known recorded use of indigo as a color name in [[English language|English]] was in 1289.<ref>Maerz and Paul ''A Dictionary of Color'' New York:1930 McGraw-Hill Page 197; Color Sample of Indigo: Page 117 Plate 47 Color Sample E10</ref>
The first known recorded use of indigo as a color name in [[English language|English]] was in 1289.<ref>Maerz and Paul ''A Dictionary of Color'' New York:1930 McGraw-Hill Page 197; Color Sample of Indigo: Page 117 Plate 47 Color Sample E10</ref>

[[Isaac Newton]] regarded indigo as a color in the [[visible spectrum]], as well as one of the seven colors of the [[rainbow]]: the color between [[blue]] and [[Violet (color)|violet]]; however, sources differ as to its actual position in the [[electromagnetic spectrum]]. Later scientists have concluded that what Newton called "blue" was what is now called [[cyan]] or blue-green; and what Newton called "indigo" was what is now called blue.

In the 1980s, programmers produced a somewhat arbitrary list of color names for the X Window computer operating system, resulting in the HTML and CSS specifications issued in the 1990s using the term "indigo" for a dark purple hue. This has resulted in violet and purple hues also being associated with the term "indigo" since that time.

Because of the [[Abney effect]], pinpointing indigo to a specific hue value in the [[HSL and HSV|HSV color wheel]] is elusive, as a higher HSV saturation value shifts the hue towards blue. However, on the new [[CAM16|CIECAM16 standard]], the hues values around 290° may be thought of as indigo, depending on the observer.


== History ==
== History ==
[[File:Indigo plant extract sample.jpg|thumb|right|Extract of natural indigo applied to paper]]
Species of ''[[Indigofera]]'' were cultivated in East Asia, [[Egypt]], [[India]], and [[Peru]] in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BC and comes from [[Huaca Prieta]], in contemporary Peru.<ref>{{cite journal |last=Splitstoser |first=Jeffrey C. |last2=Dillehay |first2=Tom D. |last3=Wouters |first3=Jan |last4=Claro |first4=Ana |date= September 2016|title=Early pre-Hispanic use of indigo blue in Peru |url=http://advances.sciencemag.org/content/2/9/e1501623.full|journal=Science Advances |volume=2 |issue=9 |pages=e1501623 |doi=10.1126/sciadv.1501623|pmid=27652337 |pmc=5023320 }}</ref> [[Pliny the Elder]] mentions the [[Indus Valley Civilization]] as the source of the dye after which it was named. It was imported from there in small quantities via the [[Silk Road]].<ref>Robin J. H. Clark, Christopher J. Cooksey, Marcus A. M. Daniels, Robert Withnall: "Indigo, woad, and Tyrian Purple: important vat dyes from antiquity to the present", ''Endeavour'' 17/4 (1993), 191–199.</ref>


===Indigo as a dye===
The [[Ancient Greek]] term for the dye was {{lang|grc|Ἰνδικὸν φάρμακον}} ("Sindhi [[:wikt:φάρμακον|dye]]"), which, adopted to Latin as ''indicum'' and via Portuguese gave rise to the modern word ''[[:wikt:indigo|indigo]]''.<ref>[http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dfa%2Frmakon Ἰνδικός] in Henry George Liddell. Robert Scott. A Greek-English Lexicon. revised and augmented throughout by. Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford. Clarendon Press. 1940; English ''indigo'' since the 17th century, changed from 16th-century ''indico''.</ref>
{{Main|Indigo dye#History}}
[[File:Indigo plant extract sample.jpg|thumb|left|Extract of natural indigo applied to paper]]


''Indigo dye'' is a blue color, obtained from several different types of plants. The indigo plant ([[Indigofera tinctoria]]) often called "true indigo" probably produces the best results, although several others are close in color: [[Japanese indigo]] (Polygonum tinctoria), Natal indigo (''[[Indigofera arrecta]]''), Guatemalan indigo (''[[Indigofera suffruticosa]]''), Chinese indigo (''[[Persicaria tinctoria]]''), and woad ''[[Isatis tinctoria]]''.
Spanish explorers discovered an American species of indigo and began to cultivate the product in [[Guatemala]]. The English and French subsequently began to encourage indigo cultivation in their colonies in the [[West Indies]].<ref>{{cite book|last1=Pritchard|first1=James|title=In Search of Empire: The French in the Americas, 1670–1730|date=2004|publisher=Cambridge University Press|location=Cambridge|page=127}}</ref>


''Indigofera tinctoria'' and related species were cultivated in [[East Asia]], [[Egypt]], [[India]], [[Bangladesh]] and [[Peru]] in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BC and comes from [[Huaca Prieta]], in contemporary Peru.<ref>{{cite journal |last1=Splitstoser |first1=Jeffrey C. |last2=Dillehay |first2=Tom D. |last3=Wouters |first3=Jan |last4=Claro |first4=Ana |date=September 2016 |title=Early pre-Hispanic use of indigo blue in Peru |journal=Science Advances |volume=2 |issue=9 |pages=e1501623 |doi=10.1126/sciadv.1501623 |pmid=27652337 |pmc=5023320 |bibcode=2016SciA....2E1623S }}</ref> [[Pliny the Elder]] mentions India as the source of the dye after which it was named.<ref>{{cite web |title=Night of the Indigo |url=https://www.harappa.com/blog/night-indigo |website=harappa.com |access-date=20 May 2016 |archive-date=14 January 2019 |archive-url=https://web.archive.org/web/20190114153312/https://www.harappa.com/blog/night-indigo |url-status=live }}</ref> It was imported from there in small quantities via the [[Silk Road]].<ref>{{cite journal | first1=Robin J. H. | last1=Clark | first2=Christopher J. | last2=Cooksey | first3=Marcus A. M. | last3=Daniels | first4=Robert | last4=Withnall | title=Indigo, woad, and Tyrian Purple: important vat dyes from antiquity to the present | journal=Endeavour | volume=17 | issue=4 | date=1993 | pages=191–199| doi=10.1016/0160-9327(93)90062-8 }}</ref>
Blue dye can be made from two different types of plants: the indigo plant, which produces the best results, and from the woad plant ''[[Isatis tinctoria]]'', also known as pastel.<ref name=":0">{{Cite news|url=https://vanessafrance.wordpress.com/2011/05/22/getting-the-blues-the-pastel-trade-in-southwest-france/|title=Getting the blues: the pastel trade in southwest France|date=2011-05-22|work=Life on La Lune|access-date=2018-02-23|language=en-US}}</ref> For a long time woad was the main source of blue dye in Europe. Woad was replaced by true indigo as trade routes opened up, and both plant sources have now been largely replaced by synthetic dyes.


The [[Ancient Greek]] term for the dye was {{lang|grc|Ἰνδικὸν φάρμακον}} (''indikon pharmakon'', "Indian [[:wikt:φάρμακον|dye]]"), which, adopted to [[Latin]] as {{lang|la|indicum}} (a [[second declension]] noun) or ''indico'' (oblique case) and via [[Portuguese language|Portuguese]], gave rise to the modern word [[:wikt:indigo|indigo]].<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dfa%2Frmakon Ἰνδικός] {{Webarchive|url=https://web.archive.org/web/20210129153557/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dfa%2Frmakon |date=29 January 2021 }} in Henry George Liddell. Robert Scott. A Greek-English Lexicon. revised and augmented throughout by. Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford. Clarendon Press. 1940; English ''indigo'' since the 17th century, changed from 16th-century ''indico''.</ref>
== Classification as a spectral color ==
[[File:Newton's colour circle.png|thumb|Indigo is one of the colors on Newton's [[color wheel]]]]


[[File:Isatis tinctoria 003.JPG|thumb|upright=0.8|Indigo extracted from woad]]
The [[Early Modern English]] word ''indigo'' referred to the dye, not to the color (hue) itself, and ''indigo'' is not traditionally part of the basic [[color terms|color-naming system]].<ref>{{cite book|last=Ottenheimer|first=Harriet Joseph|title=The anthropology of language: an introduction to linguistic anthropology|year=2009|publisher=Wadsworth|location=Belmont, CA|isbn=978-0-495-50884-7|page=29|edition=2nd}}</ref> Modern sources place indigo in the [[electromagnetic spectrum]] between 420 and 450 nanometers,<ref name="EoF" /><ref name="Huris">{{cite web|url=http://www.huris.com/web/cog/sci/phs/phy/emr.htm|title=Spectrum of Electromagnetic Radiation ( EMR )|first=The HURIS|last=Group|website=www.huris.com}}</ref><ref name="VIBGYOR">{{cite web|url=http://www.mathworks.com/matlabcentral/fileexchange/20718-vibgyor-color-segmentation/content/VIBGYORsegmentation.m|title=VIBGYOR Color Segmentation – File Exchange – MATLAB Central|website=www.mathworks.com}}</ref> which lies on the short-wave side of [[Blue (RGB)|color wheel (RGB) blue]], towards (spectral) violet.
In early Europe the main source was from the woad plant ''Isatis tinctoria'', also known as pastel.<ref name="VanessaFrance">{{Cite news |date=22 May 2011 |title=Getting the blues: the pastel trade in southwest France |language=en-US |work=Life on La Lune |url=https://vanessafrance.wordpress.com/2011/05/22/getting-the-blues-the-pastel-trade-in-southwest-france/ |access-date=23 February 2018}}</ref> For a long time, woad was the main source of blue dye in Europe. Woad was replaced by "true indigo", as trade routes opened up. Plant sources have now been largely replaced by [[synthetic dyes]].


[[Spanish explorers]] discovered an American species of indigo and began to cultivate the product in [[Guatemala]]. The English and French subsequently began to encourage indigo cultivation in their colonies in the [[West Indies]].<ref>{{cite book|last1=Pritchard|first1=James|title=In Search of Empire: The French in the Americas, 1670–1730|date=2004|publisher=Cambridge University Press|location=Cambridge|page=127}}</ref>
However, the correspondence of this definition with colors of actual indigo dyes is disputed. Optical scientists Hardy and Perrin list indigo as between 445<ref>{{cite web |url=https://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |title=Archived copy |accessdate=2016-11-24 |deadurl=yes |archiveurl=https://web.archive.org/web/20161124160412/https://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |archivedate=24 November 2016 |df=dmy-all }}</ref> and 464&nbsp;nm wavelength,<ref>Arthur C. Hardy and Fred H. Perrin. ''[http://apps.isiknowledge.com/full_record.do?product=UA&search_mode=GeneralSearch&qid=22&SID=2EdCK2KejLbni4FJpgB&page=1&doc=1&colname=BIOSIS The Principles of Optics.]'' McGraw-Hill Book Co., Inc., New York. 1932.</ref> which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards [[azure (color)|azure]].


In North America, indigo was introduced by [[Eliza Lucas]] into colonial South Carolina, where it became the colony's second-most important [[cash crop]] (after rice).<ref>{{cite web |url=http://cwh.ucsc.edu/SocialBiog.Martin.pdf |title=Eliza Lucas Pinckney:Indigo in the Atlantic World |author=Eliza Layne Martin |access-date=2013-08-24 |archive-url=https://web.archive.org/web/20100607061823/http://cwh.ucsc.edu/SocialBiog.Martin.pdf |archive-date=2010-06-07 |url-status=dead }}</ref> Before the [[American Revolutionary War|Revolutionary War]], indigo accounted for more than one-third of the value of exports from the American colonies.<ref>[http://www.nwhm.org/Education/biography_elpinckney.html "Eliza Lucas Pinckney"] {{webarchive|url=https://web.archive.org/web/20081121015821/http://www.nwhm.org/Education/biography_elpinckney.html |date=21 November 2008 }}, ''Biographies'', National Women's History Museum, 2007, accessed 7 December 2008.</ref>
[[Isaac Newton]] introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of [[Dispersive prism|prisms]] at a fair near [[Cambridge]], the [[East India Company]] had begun importing indigo dye into England,<ref>{{cite book|last=Allen|first=O.N. Allen & Ethel K.|title=The Leguminosae: a source book of characteristics, uses, and nodulation|year=1981|publisher=University of Wisconsin Press|location=Madison, Wisc.|isbn=978-0-299-08400-4|page=343|edition=null}}</ref> supplanting the homegrown [[woad]] as source of blue dye. In a pivotal experiment in the [[history of optics]], [[Isaac Newton's early life and achievements#Newton's theory of colour|the young Newton]] shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this [[optical spectrum]], Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."<!--Use the archaic spellings (blew, orang, indico, ...) from the original text per WP:SPELLING--><ref>Newton's draft of ''[http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 A Theory Concerning Light and Colors]'' on newtonproject.sussex.ac.uk</ref> He linked the seven prismatic colors to the seven notes of a western [[major scale]],<ref>{{cite web |url=http://www1.umn.edu/ships/updates/newton1.htm |title=Archived copy |accessdate=2010-10-16 |deadurl=yes |archiveurl=https://web.archive.org/web/20140929225102/http://www1.umn.edu/ships/updates/newton1.htm |archivedate=29 September 2014 |df=dmy-all }}</ref> as shown in his color wheel, with orange and indigo as the [[semitone]]s. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:
[[File:Newton prismatic colours.JPG|thumb|left|350px|Newton's observation of prismatic colors. Comparing this to a color image of the visible light spectrum will show that indigo corresponds to [[blue]], while blue corresponds to [[cyan]].]]
<blockquote>I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks.<ref>{{cite book|last=Brewster|first=David|title=Memoirs of the life, writings and discoveries of Sir Isaac Newton, Volume 1|year=1855|page=408|url=https://books.google.com/books?id=4hY6AAAAcAAJ&pg=PA408&dq=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room#v=onepage&q=For%20some%20years%20past%2C%20the%20prismatic%20colours%20being%20in%20a%20well%20darkened%20room&f=false}}</ref></blockquote>
[[File:Rainbow-diagram-ROYGBIV.svg|thumb|left|190px|Traditional seven colors of the rainbow]]
Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonic ''[[Roy G. Biv]]''. [[James Clerk Maxwell]] and [[Hermann von Helmholtz]] accepted indigo as an appropriate name for the color flanking violet in the spectrum.<ref name=ronchi>{{cite book|last=Ronchi|first=Lucia R.|title=The Excentric Blue. An Abridged Historical Review|year=2009|publisher=Fondazione Giorgio Ronchi|isbn=978-88-88649-19-1|url=https://books.google.com/books?id=E3rJnK0OUawC&pg=PA51&dq=Maxwell+and+von+Helmholtz+accepted+indigo#v=onepage&q=Maxwell%20and%20von%20Helmholtz%20accepted%20indigo&f=false|author2=Jodi Sandford}}</ref>


=== Isaac Newton's classification of indigo as a spectral color ===
Later scientists conclude that Newton named the colors differently from current usage.<ref>{{cite book|last=Evans|first=Ralph M.|title=The perception of color|year=1974|publisher=Wiley-Interscience|location=New York|isbn=978-0-471-24785-2|edition=null}}</ref><ref>{{cite journal|last=McLaren|first=K.|title=Newton's indigo|journal=Color Research & Application|date=March 2007|volume=10|issue=4|pages=225–∠229|doi=10.1002/col.5080100411}}</ref>
[[File:Newton's colour circle.png|thumb|Indigo is one of the colors on Newton's [[color wheel]].]]
According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name [[blue-green]], [[cyan]] or [[light blue]]."<ref>{{cite book|last=Waldman|first=Gary|title=Introduction to light : the physics of light, vision, and color|year=2002|publisher=Dover Publications|location=Mineola|isbn=978-0-486-42118-6|pages=193|url=https://books.google.com/books?id=PbsoAXWbnr4C&pg=PA193&dq=Newton+color+Indigo#v=onepage&q=Newton%20color%20Indigo&f=false|edition=Dover}}</ref> If this is true, Newton's seven spectral colors would have been:
<center>[[Red]]:{{color box|#ff0000}} [[Orange (colour)|Orange]]:{{color box|#ff7f00}} [[Yellow]]:{{color box|#ffff00}} [[Green]]:{{color box|#00ff00}} [[Blue]]:{{color box|#54baff}} Indigo:{{color box|#0000ff}} [[Violet (color)|Violet]]:{{color box|#9F00FF}}</center>


Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of [[Dispersive prism|prisms]] at a fair near [[Cambridge]], the [[East India Company]] had begun importing indigo dye into England,<ref>{{cite book|last=Allen|first=O.N. Allen & Ethel K.|title=The Leguminosae: a source book of characteristics, uses, and nodulation|year=1981|publisher=University of Wisconsin Press|location=Madison, Wisc.|isbn=978-0-299-08400-4|page=343|edition=null}}</ref> supplanting the homegrown [[woad]] as source of blue dye. In a pivotal experiment in the [[history of optics]], [[Isaac Newton's early life and achievements#Newton's theory of colour|the young Newton]] shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this [[optical spectrum]], Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."<!--Use the archaic spellings (blew, orang, indico, ...) from the original text per WP:SPELLING--><ref>Newton's draft of ''[http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 A Theory Concerning Light and Colors] {{Webarchive|url=https://web.archive.org/web/20110721021035/http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 |date=21 July 2011 }}'' on newtonproject.sussex.ac.uk</ref> He linked the seven prismatic colors to the seven notes of a western [[major scale]],<ref>{{cite web |url=http://www1.umn.edu/ships/updates/newton1.htm |title=SHiPS Resource Center &#124;&#124; Newton's Colors |access-date=16 October 2010 |url-status=dead |archive-url=https://web.archive.org/web/20140929225102/http://www1.umn.edu/ships/updates/newton1.htm |archive-date=29 September 2014 }}</ref> as shown in his color wheel, with orange and indigo as the [[semitone]]s. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:
The human eye does not readily differentiate [[hue]]s in the wavelengths between what we today call blue and violet. If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to [[Isaac Asimov]], "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes it seems merely deep blue."<ref>{{cite book|last=Asimov|first=Isaac|title=Eyes on the universe : a history of the telescope|year=1975|publisher=Houghton Mifflin|location=Boston|isbn=978-0-395-20716-1|page=59}}</ref>
[[File:Newton prismatic colours.JPG|thumb|left|upright=1.59|Newton's observation of prismatic colors: Comparing this to a color image of the visible light spectrum shows that "blue" corresponds to [[cyan]], while "indigo" corresponds to [[blue]].]]
<blockquote>I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks.<ref>{{cite book|last=Brewster|first=David|title=Memoirs of the life, writings and discoveries of Sir Isaac Newton, Volume 1|year=1855|page=408|url=https://books.google.com/books?id=4hY6AAAAcAAJ&q=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room&pg=PA408|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145344/https://books.google.com/books?id=4hY6AAAAcAAJ&q=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room&pg=PA408|url-status=live}}</ref></blockquote>
[[File:Rainbow-diagram-ROYGBIV.svg|thumb|left|upright=0.86|Traditional seven colors of the rainbow]]
Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonics "Richard of York gave battle in vain" and ''[[Roy G. Biv]]''. [[James Clerk Maxwell]] and [[Hermann von Helmholtz]] accepted indigo as an appropriate name for the color flanking violet in the spectrum.<ref name="ronchi">{{cite book|last=Ronchi|first=Lucia R.|title=The Excentric Blue. An Abridged Historical Review|year=2009|publisher=Fondazione Giorgio Ronchi|isbn=978-88-88649-19-1|url=https://books.google.com/books?id=E3rJnK0OUawC&q=Maxwell+and+von+Helmholtz+accepted+indigo&pg=PA51|author2=Jodi Sandford|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145347/https://books.google.com/books?id=E3rJnK0OUawC&q=Maxwell%20and%20von%20Helmholtz%20accepted%20indigo&pg=PA51|url-status=live}}</ref>


Later scientists concluded that Newton named the colors differently from current usage.<ref>{{cite book|last=Evans|first=Ralph M.|title=The perception of color|year=1974|publisher=Wiley-Interscience|location=New York|isbn=978-0-471-24785-2|edition=null}}</ref><ref>{{cite journal|last=McLaren|first=K.|title=Newton's indigo|journal=Color Research & Application|date=March 2007|volume=10|issue=4|pages=225–∠229|doi=10.1002/col.5080100411}}</ref>
Modern [[colorimetry|color scientists]] typically divide the spectrum between violet and blue at about 450&nbsp;nm, with no indigo.<ref name=hunt>{{cite book | title = Measuring Color | author = J. W. G. Hunt | year = 1980 | publisher = Ellis Horwood Ltd | isbn = 978-0-7458-0125-4}}</ref><ref>{{cite book | url = https://books.google.com/books?id=1oDOWr_yueIC&pg=PA214&lpg=PA214&dq=indigo+spectra+blue+violet+date:1990-2007#PPA214,M1 | title = Fundamentals of Atmospheric Radiation | author = Craig F. Bohren and Eugene E. Clothiaux |publisher = Wiley-VCH | year = 2006 | isbn = 978-3-527-40503-9}}</ref>
According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name [[blue-green]] or [[cyan]]."<ref name=Waldman2002>{{cite book|last=Waldman|first=Gary|title=Introduction to light : the physics of light, vision, and color|date=1983|page=193|edition=2002 Dover revised|publisher=Dover Publications|location=Mineola|isbn=978-0-486-42118-6|url=https://books.google.com/books?id=PbsoAXWbnr4C&q=Newton+color+Indigo&pg=PA193}}</ref> If this is true, Newton's seven spectral colors would have been:
{{center|[[Red]]:{{color box|#ff0000}} [[Orange (colour)|Orange]]:{{color box|#ff8000}} [[Yellow]]:{{color box|#ffff00}} [[Green]]:{{color box|#00ff00}} [[Blue]]:{{color box|#54baff}} Indigo:{{color box|#0000ff}} [[Violet (color)|Violet]]:{{color box|#8B00FF}}}}


The human eye does not readily differentiate hues in the wavelengths between what are now called blue and violet. If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to [[Isaac Asimov]], "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes, it seems merely deep blue."<ref>{{cite book|last=Asimov|first=Isaac|title=Eyes on the universe : a history of the telescope|year=1975|publisher=Houghton Mifflin|location=Boston|isbn=978-0-395-20716-1|page=[https://archive.org/details/eyesonuniverse00isaa/page/59 59]|url-access=registration|url=https://archive.org/details/eyesonuniverse00isaa/page/59}}</ref>
== Distinction among the four major tones of indigo ==
Like many other colors ([[Orange (colour)|orange]], [[Rose (color)|rose]], and [[violet (color)|violet]] are the best-known), indigo gets its name from an object in the natural world—the plant named [[indigo plant|indigo]] once used for dyeing cloth (see also [[Indigo dye]]).


===1800s===
The color ''electric indigo'' is a bright and [[colorfulness|saturated]] color between the traditional indigo and violet. This is the [[lightness|brightest]] color indigo that can be approximated on a computer screen; it is a color located between the (primary) blue and the color [[Violet (color)#Electric violet|violet]] of the RGB color wheel.
In 1821, [[Abraham Werner]] published ''Werner's Nomenclature of Colours'', where indigo, called ''indigo blue'', is classified as a blue hue, and not listed among the violet hues. He writes that the color is composed of "[[Prussian blue|Berlin blue]], a little black, and a small portion of apple green," and indicating it is the color of blue [[copper ore]], with Berlin blue being described as the color of a [[blue jay]]'s wing, a [[hepatica]] flower, or a blue [[sapphire]].<ref name="Werner">{{cite book |last1=Werner |first1=Abraham |title=Werner's Nomenclature of Colours |date=1821 |location=London |page=41 |url=https://archive.org/details/gri_c00033125012743312/page/n41/mode/2up|access-date=30 October 2023}}</ref>


According to an article, ''Definition of the Color Indigo'' published in ''[[Nature (journal)|Nature]]'' magazine in the late 1800s, Newton's use of the term "indigo" referred to a spectral color between blue and violet. However, the article states that Wilhelm von Bezold, in his treatise on color, disagreed with Newton's use of the term, on the basis that the pigment indigo was a darker hue than the spectral color; and furthermore, Professor [[Ogden Rood]] points out that indigo pigment corresponds to the cyan-blue region of the spectrum, lying between blue and green, although darker in hue. Rood considers that artificial [[ultramarine]] pigment is closer to the point of the spectrum described as "indigo", and proposed renaming that spectral point as "ultramarine". The article goes on to state that comparison of the pigments, both dry and wet, with Maxwell's discs and with the spectrum, that indigo is almost identical to [[Prussian blue]], stating that it "certainly does not lie on the violet side of 'blue.'" When scraped, a lump of indigo pigment appears more violet, and if powdered or dissolved, becomes greenish.<ref name="Nature">{{cite journal |title=Definition of the Color Indigo |journal=Littel's Living Age |date=1880 |volume=145 |issue=1869 |url=https://en.wikisource.org/wiki/Littell%27s_Living_Age/Volume_145/Issue_1869/Definition_of_the_Color_Indigo |access-date=30 October 2023 |quote=Newton denoted by the name of "indigo" the tint of the spectrum lying between "blue" and "violet." Von Bezold, in his work on color, rejects the term, justifying his objection by observing that the pigment indigo is a much darker hue than the spectrum tint. Prof. O. N. Rood, who follows Von Bezold in rejecting the term, brings forward the further objection that the tint of the pigment indigo more nearly corresponds in hue (though it is darker) with the cyan-blue region lying between green and blue. By comparing the tints of indigo pigment, both dry and wet, with the spectrum, and by means of Maxwell’s disks, it appears that the hue of indigo is almost identical with that of Prussian blue, and certainly does not lie on the violet side of "blue." Indigo in the dry lump, if scraped, has, however, a more violet tint; but if fractured or powdered, or dissolved, its tint is distinctly greenish. Prof. Rood considers that artificial ultramarine corresponds much more nearly to the true tint of the spectrum at the point usually termed "indigo," and he therefore proposes to substitute the term "ultramarine" in its place, the color of the artificial pigment being thereby intended. |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030033534/https://en.wikisource.org/wiki/Littell%27s_Living_Age/Volume_145/Issue_1869/Definition_of_the_Color_Indigo |url-status=live }}</ref>
The web color ''blue violet'' or ''deep indigo'' is a tone of indigo brighter than pigment indigo, but not as bright as electric indigo.


===Modern spectral classification===
The color ''pigment indigo'' is equivalent to the web color indigo and approximates the color indigo that is usually reproduced in pigments and colored pencils.
Several modern sources place indigo in the [[electromagnetic spectrum]] between 420 and 450 nanometers,<ref name="EoF" /><ref name="Huris">{{cite web|url=http://www.huris.com/web/cog/sci/phs/phy/emr.htm|title=Spectrum of Electromagnetic Radiation ( EMR )|first=The HURIS|last=Group|website=www.huris.com|access-date=14 June 2013|archive-date=28 January 2020|archive-url=https://web.archive.org/web/20200128045739/http://www.huris.com/web/cog/sci/phs/phy/emr.htm|url-status=dead}}</ref><ref name="VIBGYOR">{{cite web|url=http://www.mathworks.com/matlabcentral/fileexchange/20718-vibgyor-color-segmentation/content/VIBGYORsegmentation.m|title=VIBGYOR Color Segmentation – File Exchange – MATLAB Central|website=www.mathworks.com|date=June 2023|access-date=14 June 2013|archive-date=19 October 2013|archive-url=https://web.archive.org/web/20131019103723/http://www.mathworks.com/matlabcentral/fileexchange/20718-vibgyor-color-segmentation/content/VIBGYORsegmentation.m|url-status=live}}</ref> which lies on the short-wave side of [[Shades of blue#Blue (RGB) (X11 blue)|color wheel (RGB) blue]], towards (spectral) violet.

The correspondence of this definition with colors of actual indigo dyes, though, is disputed. Optical scientists Hardy and Perrin list indigo as between 445<ref>{{cite web |url=https://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |title=What Wavelength Goes with a Color? |access-date=24 November 2016 |url-status=dead |archive-url=https://web.archive.org/web/20161124160412/https://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |archive-date=24 November 2016 }}</ref> and 464&nbsp;nm wavelength,<ref>Arthur C. Hardy and Fred H. Perrin. ''[http://apps.isiknowledge.com/full_record.do?product=UA&search_mode=GeneralSearch&qid=22&SID=2EdCK2KejLbni4FJpgB&page=1&doc=1&colname=BIOSIS The Principles of Optics.]'' McGraw-Hill Book Co., Inc., New York. 1932.</ref> which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards [[azure (color)|azure]].

Other modern [[colorimetry|color scientists]], such as Bohren and Clothiaux (2006), and J.W.G. Hunt (1980), divide the spectrum between violet and blue at about 450&nbsp;nm, with no hue specifically named indigo.<ref name=Bohren>{{cite book | url = https://books.google.com/books?id=1oDOWr_yueIC&q=indigo+spectra+blue+violet+date:1990-2007&pg=PA214 | title = Fundamentals of Atmospheric Radiation | author = Craig F. Bohren and Eugene E. Clothiaux | publisher = Wiley-VCH | year = 2006 | isbn = 978-3-527-40503-9 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name=hunt>{{cite book | title = Measuring Color | author = J. W. G. Hunt | year = 1980 | publisher = Ellis Horwood Ltd | isbn = 978-0-7458-0125-4}}</ref>

===Web era===
====Origin of "Indigo" as a name for purple in web pages====
Towards the end of the 20th century, purple colors also became referred to as "indigo". In the 1980s, computer programmers [[Jim Gettys]], Paul Ravelling, John C. Thomas and Jim Fulton produced a list of colors for the [[X Window System|X Window]] Operating System. The color identified as "indigo" was not the color indigo (as generally understood at the time), but was actually a dark purple hue; the programmers assigned it the [[Web colors| hex code]] #4B0082 {{Colorsample|#4B0082|1}}. This collection of color names was somewhat arbitrary: Thomas used a box of 72 [[Crayola]] crayons as a standard, whereas Ravelling used color swabs from the now-defunct Sinclair Paints company, resulting in the color list for version [[X11_color_names#Color_name_chart|X11 ]] of the operating system containing fanciful color names such as "papaya whip", "blanched almond" and "peach puff". The database was also criticised for its many inconsistencies, such as "dark gray" being lighter than "gray", and for the color distribution being uneven, tending towards reds and greens at the expense of blues.<ref>{{cite news | url=https://arstechnica.com/information-technology/2015/10/tomato-versus-ff6347-the-tragicomic-history-of-css-color-names/ | title="Tomato" versus "#FF6347"—the tragicomic history of CSS color names | work=Ars Technica | date=October 11, 2015 | access-date=October 11, 2015 | first=Julianne | last=Tveten | archive-date=29 October 2023 | archive-url=https://web.archive.org/web/20231029182142/https://arstechnica.com/information-technology/2015/10/tomato-versus-ff6347-the-tragicomic-history-of-css-color-names/ | url-status=live }}</ref>

In the 1990s, this list which came with version X11 became the basis of the [[HTML]] and [[CSS]] color rendition used in websites and web design. This resulted in the name "Indigo" being associated with purple and violet hues in web page design and graphic design. Physics author John Spacey writes on the website ''Simplicable'' that the X11 programmers did not have any background in color theory, and that as these names are used by web designers and graphic designers, the name ''indigo'' has since that time been strongly associated with purple or violet. Spacey writes, "As such, a few programmers accidentally repurposed a color name that was known to civilisations for thousands of years."<ref name="Spacey">{{cite web |last1=Spacey |first1=John |title=19 Types of Indigo |date=19 June 2020 |url=https://simplicable.com/colors/indigo |website=Simplicable |access-date=30 October 2023 |archive-url=https://web.archive.org/web/20230605223840/https://simplicable.com/colors/indigo |archive-date=2023-06-05 |quote=In 1986 some programmers created a list of color names for a unix system known as X11. Having no background in color theory, they placed indigo as a dark purple. This list was later used by HTML and CSS standards that remain in place to this day. These standards are used by millions of designers and digital artists such that the color name indigo is now strongly associated with dark purple or violet. As such, a few programmers accidentally repurposed a color name that was known to civilizations for thousands of years. ...Note the difference between Web Indigo and Indigo. This standard color name is completely detached from the traditional color. This misrepresentation resulted from the random selection by a programmer working on an operating system in 1986. |url-status=live}}</ref>

====Crayola crayon colors====
The Crayola company released an [[Crayola#Colors_chart|indigo crayon]] in 1999, with the Crayola website using the [[hex code]] #4F49C6 {{Colorsample|#4F49C6|1}} to [[List of Crayola crayon colors|approximate the crayon color]]. The 2001 iron indigo crayon is portrayed using hex code #184FA1 {{Colorsample|#184FA1|1}}, the 2004 indigo crayon color uses #5D76CB {{Colorsample|#5D76CB|1}}, the 2019 iridescent indigo uses #3C32CD {{Colorsample|#3C32CD|1}}.

== Distinction among tones of indigo ==
Like many other colors ([[Orange (colour)|orange]], [[Rose (color)|rose]], and [[violet (color)|violet]] are the best-known), indigo gets its name from an object in the natural world—the plant named [[indigo plant|indigo]] once used for dyeing cloth (see also [[Indigo dye]]).

The color pigment indigo is equivalent to the web color indigo and approximates the color indigo that is usually reproduced in pigments and colored pencils.


The color of indigo dye is a different color from either spectrum indigo or pigment indigo. This is the actual color of the dye. A vat full of this dye is a darker color, approximating the web color [[midnight blue]].
The color of indigo dye is a different color from either spectrum indigo or pigment indigo. This is the actual color of the dye. A vat full of this dye is a darker color, approximating the web color [[midnight blue]].


The color "electric indigo" is a bright and [[colorfulness|saturated]] color between the traditional indigo and violet. This is the [[lightness|brightest]] color indigo that can be approximated on a computer screen; it is a color located between the (primary) blue and the color [[Violet (color)#Electric violet|violet]] of the RGB color wheel.
Below are displayed these four major tones of indigo.

The web color blue violet or deep indigo is a tone of indigo brighter than pigment indigo, but not as bright as electric indigo.

Listed below are several indigo hues, some of which have included the word "indigo", with the adoption of [[HTML color names]] in the [[World Wide Web]] era.

=== Indigo dye color ===
{{Main|Indigo dye}}
{{infobox color
|title=Indigo Dye
|hex=00416A
|source=[http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm]
|isccname=Deep blue}}
''Indigo dye'' is a greenish dark blue color, obtained from either the leaves of the tropical Indigo plant (''[[Indigofera]]''), or from woad (''[[Isatis tinctoria]]''), or the Chinese indigo (''[[Persicaria tinctoria]]''). Many societies make use of the ''[[Indigofera]]'' plant for producing different shades of blue. Cloth that is repeatedly boiled in an indigo dye bath-solution (boiled and left to dry, boiled and left to dry, etc.), the blue pigment becomes darker on the cloth. After dyeing, the cloth is hung in the open air to dry.

A Native American woman described the process used by the [[Cherokee Indians]] when extracting the dye:
<blockquote>We raised our indigo which we cut in the morning while the dew was still on it; then we put it in a tub and soaked it overnight, and the next day we foamed it up by beating it with a gourd. We let it stand overnight again, and the next day rubbed [[tallow]] on our hands to kill the foam. Afterwards, we poured the water off, and the sediment left in the bottom we would pour into a pitcher or crock to let it get dry, and then we would put it into a poke made of cloth (i.e. sack made of coarse cloth) and then when we wanted any of it to dye [there]with, we would take the dry indigo.<ref>{{Citation |contribution=History of the Cherokees, 1830–1846 |title=Chronicles of Oklahoma |last1=Knight |first1=Oliver |publisher=Oklahoma Historical Society|place=Oklahoma City|year=1956–57|page=164|oclc=647927893 |language=en }}</ref><ref>{{cite book|author-last=Foreman|author-first=Grant|title=The Five Civilized Tribes|publisher=University of Oklahoma Press|location=Norman|year=1934|page=283|language=en|url=https://books.google.com/books?id=gGtLnSkqkekC&q=We+raised+our+indigo&pg=PA283|isbn=978-0-8061-0923-7|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145344/https://books.google.com/books?id=gGtLnSkqkekC&q=We+raised+our+indigo&pg=PA283|url-status=live}}</ref></blockquote>

In [[Sa Pa]], Vietnam, the tropical Indigo (''Indigo tinctoria'') leaves are harvested and, while still fresh, placed inside a tub of room-temperature to lukewarm water where they are left to sit for 3 to 4 days and allowed to ferment, until the water turns green. Afterwards, crushed limestone ([[pickling lime]]) is added to the water, at which time the water with the leaves are vigorously agitated for 15 to 20 minutes, until the water turns blue. The blue pigment settles as sediment at the bottom of the tub. The sediment is scooped out and stored. When dyeing cloth, the pigment is then boiled in a vat of water; the cloth (usually made from yarns of [[hemp]]) is inserted into the vat for absorbing the dye. After hanging out to dry, the boiling process is repeated as often as needed to produce a darker color.
{{Clear}}

=== Indigo (color wheel) ===
{{infobox color
| title=Indigo (color wheel)
| hex=4000FF
| hexref=<ref name="electric-coords">{{cite web |title=What Color is Indigo? |url=http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm |url-status=dead |date=16 December 2014 |archive-url=https://web.archive.org/web/20151226130706/http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm |archive-date=26 December 2015}}</ref>
|isccname=Vivid purplish blue}}

In a [[RGB color space]], "Indigo(color wheel)" is composed of 25.1% [[red]], 0% [[green]] and 100% [[blue]]. Whereas in a CMYK color space, it is composed of 74.9% [[cyan]], 100% [[magenta]], 0% [[yellow]] and 0% [[black]]. It has a hue angle of 255.1 degrees, a saturation of 100% and a lightness of 50%. Indigo(color wheel) could be obtained by blending [[Violet (color)#Violet(color wheel)|violet]] with [[blue]].
{{Clear}}


=== Electric indigo ===
=== Electric indigo ===
{{infobox color
{{infobox color
| title=Electric Indigo|textcolor=white
| title=Electric Indigo
| hex=6F00FF
| hex=6F00FF
| hexref=<ref name="electric-coords">{{cite web |title=What Color is Indigo? |url=http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm |url-status=dead |date=16 December 2014 |archive-url=https://web.archive.org/web/20151226130706/http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm |archive-date=26 December 2015}}</ref>
| r=111|g= 0|b=255
|isccname=Vivid purplish blue}}
| c= 57|m=100|y= 0|k= 0
| h=266|s=100|v=100<ref>{{cite web|url=http://web.forret.com/tools/color.asp?RGB=#6F00FF|title=RGB Color converter – toolstudio|first=Peter|last=Forret|website=web.forret.com}}</ref>
|source=[http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm]}}


"Electric indigo" is brighter than the pigment indigo reproduced below. When plotted on the [[CIE chromaticity diagram]], this color is at 435 nanometers, in the middle of the portion of the spectrum traditionally considered indigo, i.e., between 450 and 420 nanometers. This color is only an approximation of spectral indigo, since actual spectral colors are outside the [[gamut]] of the [[sRGB]] color system.
"Electric indigo" is brighter than the pigment indigo reproduced above. When plotted on the [[CIE chromaticity diagram]], this color is at 435 nanometers, in the middle of the portion of the spectrum traditionally considered indigo, i.e., between 450 and 420 nanometers. This color is only an approximation of spectral indigo, since actual spectral colors are outside the [[gamut]] of the [[sRGB]] color system.
{{Clear}}
{{Clear}}


=== Deep indigo (web color blue-violet) ===
=== Deep indigo (web color blue-violet) ===
{{infobox color
{{infobox color
| title=Blue-Violet|textcolor=white
|title=Blue-Violet
| hex=8A2BE2
|hex=8A2BE2
|source=[[Web colors#X11 color names|X11]]
| r=138|g= 43|b=226
|isccname=Vivid violet}}
| c= 63|m= 81|y= 0|k= 0
| h=271|s= 81|v= 89|
|source=[[Web colors#X11 color names|X11]]}}


At right is displayed the web color "blue-violet", a color intermediate in brightness between electric indigo and pigment indigo. It is also known as "deep indigo".
At right is displayed the web color "blue-violet", a color intermediate in brightness between electric indigo and pigment indigo. It is also known as "deep indigo".
Line 99: Line 144:
=== Web color indigo ===
=== Web color indigo ===
{{infobox color
{{infobox color
| title=Web color Indigo|textcolor=white
|title=Web color Indigo
| hex= 4B0082
|hex=4B0082
|source=[https://www.colorhexa.com/4b0082]
| r= 75|g= 0|b=130
|isccname=Dark violet}}
| c= 42|m=100|y= 0|k= 49

| h=247.6|s=100|v=51
{{infobox color
|source=[https://www.colorhexa.com/4b0082]}}
|title=Web safe Indigo
The color box at right displays the web color indigo, the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that is possible to reproduce on a computer screen. Its hue is closer to violet than to indigo dye for which the color is named. Pigment indigo can be obtained by mixing 55% pigment cyan with about 45% pigment [[magenta]].
|hex=330099
|source=[https://rgbcolorcode.com/color/330099]
|isccname=}}

The color box on the right displays the web color indigo, the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that is possible to reproduce on a computer screen. Its hue is closer to violet than to indigo dye for which the color is named. Pigment indigo can be obtained by mixing 55% pigment cyan with about 45% pigment [[magenta]].


Compare the subtractive colors to the additive colors in the two primary color charts in the article on [[primary colors]] to see the distinction between electric colors as reproducible from light on a computer screen (additive colors) and the pigment colors reproducible with pigments (subtractive colors); the additive colors are significantly brighter because they are produced from light instead of pigment.
Compare the subtractive colors to the additive colors in the two primary color charts in the article on [[primary colors]] to see the distinction between electric colors as reproducible from light on a computer screen (additive colors) and the pigment colors reproducible with pigments (subtractive colors); the additive colors are significantly brighter because they are produced from light instead of pigment.


Web color indigo represents the way the color indigo was always reproduced in pigments, paints, or colored [[pencil]]s in the 1950s. By the 1970s, because of the advent of [[psychedelic art]], artists became used to brighter pigments, and pigments called "bright indigo" or "bright blue-violet" that are the pigment equivalent of the electric indigo reproduced in the section above became available in artists' pigments and colored pencils.
Web color indigo represents the way the color indigo was always reproduced in pigments, paints, or colored [[pencil]]s in the 1950s. By the 1970s, because of the advent of [[psychedelic art]], artists became accustomed to brighter pigments. Pigments called "bright indigo" or "bright blue-violet" (the pigment equivalent of the electric indigo reproduced in the section above) became available in artists' pigments and colored pencils.
{{Clear}}
{{Clear}}


=== Tropical indigo ===
=== Tropical indigo ===
{{infobox color|title=Tropical Indigo
{{infobox color
|title=Tropical Indigo
|hex=9683EC
|hex=9683EC
|source=Gallego and Sanz<ref>Gallego, Rosa; Sanz, Juan Carlos (2005). ''Guía de coloraciones'' (Gallego, Rosa; Sanz, Juan Carlos (2005). ''Guide to Colorations'') Madrid: H. Blume. {{ISBN|84-89840-31-8}}</ref>
|textcolor=white
|isccname=Vivid violet}}
|r=150
|g=131
|b=236
|c=48
|m=51
|y=0
|k=0
|h=251
|s=44
|v=93
|source=Gallego and Sanz<ref>Gallego, Rosa; Sanz, Juan Carlos (2005). ''Guía de coloraciones'' (Gallego, Rosa; Sanz, Juan Carlos (2005). ''Guide to Colorations'') Madrid: H. Blume. {{ISBN|84-89840-31-8}}</ref>}}


'Tropical Indigo' is the color that is called ''añil'' in the ''Guía de coloraciones'' (Guide to colorations) by Rosa Gallego and
'Tropical Indigo' is the color that is called ''añil'' in the ''Guía de coloraciones'' (Guide to colorations) by Rosa Gallego and
Line 132: Line 173:
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{{Clear}}


=== Indigo dye ===
===Imperial blue===
{{infobox color
{{infobox color
| title=Indigo Dye|textcolor=white
| title=Imperial Blue
| hex=00416a
| hex=002395
| isccname=Vivid blue}}
| r=0|g= 65|b=106
| c= 100|m=39|y= 0|k= 58
| h=203|s=100|v=42
|source=[http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm]}}
''Indigo dye'' is a greenish dark blue color.
{{Clear}}
{{Clear}}


== In nature ==
== In nature ==
;[[Birds]]
===Birds===
[[File:Indigo Bunting.jpg|alt=Indigo bunting|thumb|[[Indigo bunting]]]]
: Male [[indigobird]]s are a very dark, metallic blue.
: The [[indigo bunting]], native to North America, is mostly bright [[cerulean]] blue with an indigo head.
* Male [[indigobird]]s are a very dark, metallic blue.
* The [[indigo bunting]], native to North America, is mostly bright [[cerulean]] blue with an indigo head.
: The related [[blue grosbeak]] is, ironically, more indigo than the indigo bunting.
* The related [[blue grosbeak]] is, ironically, more indigo than the indigo bunting.
;[[Fungi]]
: ''[[Lactarius indigo]]'' is one of the very few species of [[mushroom]]s colored in tones of blue.
;[[Snakes]]
: The [[eastern indigo snake]], ''Drymarchon couperi'', of the southeastern United States, is a dark blue/black.


== In culture ==
===Fungi===
[[File:Lactarius indigo 48568 edit.jpg|alt=An upturned Lactarius indigo mushroom|thumb|An upturned ''[[Lactarius indigo]]'' mushroom]]
'''<big>Literature</big>'''
* ''[[Lactarius indigo]]'' is one of the very few species of [[mushroom]]s colored in tones of blue.


===Snakes===
Marina Warner's novel ''[[Indigo (Warner novel)|Indigo]]'' (1992) is a retelling of Shakespeare's ''The Tempest'' and features the production of indigo dye by Sycorax.
[[File:Eastern Indigo Snake.jpg|alt=Eastern indigo snake|thumb|[[Eastern indigo snake]]]]
* The [[eastern indigo snake]], ''Drymarchon couperi'', of the southeastern United States, is a dark blue/black.


== In culture ==
=== Business ===
=== Business ===
* [[IndiGo Airlines]] is an Indian budget airline that uses an indigo logo and operates only [[Airbus A320]]s.
* [[IndiGo]] is an Indian budget airline that uses an indigo logo and operates only [[Airbus A320]]s.
* [[Indigo Books and Music]] uses an indigo logo and has sometimes referred to the color as "blue" in advertising.<ref>"It's New and It's Blue" (Indigo advertisement), ''[[The Globe and Mail]]'', Toronto, October&nbsp;1, 1999, p.&nbsp;A3</ref><ref>"Indigo Bookstore had a 'Think Blue' campaign back in 1999" according to: {{cite web|title=Think Blue 2008: a Before and After Tale of Silly Turf Battles and Redemptive Communication|url=http://www.irregulartimes.com/thinkblue2008.html|accessdate=2013-02-04}}{{better source|date=February 2013}}</ref>
* [[Indigo Books and Music]] uses an indigo logo and has sometimes referred to the color as "blue" in advertising.<ref>"It's New and It's Blue" (Indigo advertisement), ''[[The Globe and Mail]]'', Toronto, 1 October 1999, p.&nbsp;A3</ref><ref>"Indigo Bookstore had a 'Think Blue' campaign back in 1999" according to: {{cite web|title=Think Blue 2008: a Before and After Tale of Silly Turf Battles and Redemptive Communication|url=http://www.irregulartimes.com/thinkblue2008.html|access-date=4 February 2013|archive-date=10 May 2012|archive-url=https://web.archive.org/web/20120510202417/http://www.irregulartimes.com/thinkblue2008.html|url-status=dead}}{{better source|date=February 2013}}</ref>
* The [[GameCube]] was initially released in 2 color variants, including one bearing the title of 'Indigo', with the main console and controllers in that color.
* [[Indigo Dreams]] is an award-winning publishing company based in Devon, England.
* [[Indigo Line]], a proposed [[MBTA]] [[commuter rail]] line, is set to open in 2024.


=== Computer graphics ===
=== Computer graphics ===
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=== Dyes ===
=== Dyes ===
{{further|Indigo dye#History of indigo}}
[[File:COLLECTIE TROPENMUSEUM Indigo wordt aangemaakt in kuilen in uitgehakte puimsteentufgrond Karolanden TMnr 10014190.jpg|thumb|Indigo is created in potholes carved in pumice "tufgrond" in Karoland, [[Sumatra]]]]
[[File:COLLECTIE TROPENMUSEUM Indigo wordt aangemaakt in kuilen in uitgehakte puimsteentufgrond Karolanden TMnr 10014190.jpg|thumb|Indigo is created in potholes carved in pumice "tufgrond" in [[Karo Regency|Karoland]], [[Sumatra]].]]
* [[Guatemala]], as of 1778, was considered one of the world's foremost providers of indigo.<ref name=Kitchin>{{cite book|last=Kitchin|first=Thomas|title=The Present State of the West-Indies: Containing an Accurate Description of What Parts Are Possessed by the Several Powers in Europe|year=1778|publisher=R. Baldwin|location=London|page=30|url=http://www.wdl.org/en/item/4397/view/1/30/}}</ref>
* Indigo dye was used to dye denim, giving the original '[[Jeans|blue jeans]]' their distinctive colour.
* In Mexico, indigo is known as 'añil'.<ref>Gallego, Rosa; Sanz, Juan Carlos (2001). Diccionario Akal del color. Akal. {{ISBN|978-84-460-1083-8}}.</ref> After silver, and [[cochineal]] to produce red, ''añil'' was the most important product exported by historical Mexico.<ref>Article „añil“ in: ''Enciclopedia de México'', vol 1, Mexiko-City: Secretaría de Educacion Pública, 1987</ref>
* The original Postal Worker uniform contained indigo dye, partly due to the dye not running when wet. <ref>{{cite web |last1=Hejna |first1=Jamie |title=What Color is Indigo? |url=https://freelancefaucet.com/answered/what-color-is-indigo/ |website=Freelance Faucet |date=19 February 2023 |access-date=19 February 2023 |archive-date=19 February 2023 |archive-url=https://web.archive.org/web/20230219153939/https://freelancefaucet.com/answered/what-color-is-indigo/ |url-status=live }}</ref>
*[[Guatemala]], as of 1778, was considered one of the world's foremost providers of indigo.<ref name="Kitchin">{{cite book|last=Kitchin|first=Thomas|title=The Present State of the West-Indies: Containing an Accurate Description of What Parts Are Possessed by the Several Powers in Europe|year=1778|publisher=R. Baldwin|location=London|page=30|url=http://www.wdl.org/en/item/4397/view/1/30/|access-date=30 August 2013|archive-date=19 October 2013|archive-url=https://web.archive.org/web/20131019122932/http://www.wdl.org/en/item/4397/view/1/30/|url-status=live}}</ref>
* In Mexico, indigo is known as ''[[Indigofera suffruticosa|añil]]''.<ref>Gallego, Rosa; Sanz, Juan Carlos (2001). Diccionario Akal del color. Akal. {{ISBN|978-84-460-1083-8}}.</ref> After silver, and [[cochineal]] to produce red, ''añil'' was the most important product exported by historical Mexico.<ref>Article „añil“ in: ''Enciclopedia de México'', vol 1, Mexiko-City: Secretaría de Educacion Pública, 1987</ref>
* The use of ''añil'' is survived in the Philippines, particularly in the Visayas and Mindanao. The powder dye is mixed with vinegar to be applied to the cheek of a person suffering from mumps.{{cn|date=May 2019}}


=== Food ===
=== Food ===
* Scientists discovered in 2008 that when a [[banana]] becomes ripe and ready to eat, it glows bright indigo under a [[black light]]. Some [[insects]], as well as [[bat]]s and [[bird]]s, may see into the [[ultraviolet]], because they are [[Tetrachromacy|tetrachromats]] and can use this information to tell when a banana is ripe and ready to eat. The glow is the result of a chemical created as the green [[chlorophyll]] in the peel breaks down.<ref>''[[Wired (magazine)|Wired]]'' Volume 19 No. 10 October 2011 Page 50</ref>
* Scientists discovered in 2008 that when a [[banana]] becomes ripe, it glows bright indigo under a [[black light]]. Some insects, as well as birds, see into the [[ultraviolet]], because they are [[Tetrachromacy|tetrachromats]] and can use this information to tell when a banana is ready to eat. The glow is the result of a chemical created as the green [[chlorophyll]] in the peel breaks down.<ref>{{cite magazine |first= Rachel |last= Zurer |title= Three Smart Things About Banana Peels |url= https://www.wired.com/2011/09/st_3st_banana/ |access-date= 28 February 2020 |magazine= [[Wired (magazine)|Wired]] |df= mdy-all |archive-date= 28 February 2020 |archive-url= https://web.archive.org/web/20200228213507/https://www.wired.com/2011/09/st_3st_banana/ |url-status= live }}</ref>

=== Literature ===
Marina Warner's novel ''[[Indigo (Warner novel)|Indigo]]'' (1992) is a retelling of Shakespeare's ''[[The Tempest]]'' and features the production of indigo dye by Sycorax.


=== Military ===
=== Military ===
The [[French Army]] adopted dark blue indigo at the time of the [[French Revolution]], as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, [[Napoleon]] decided to restore the white coats because of shortages of [[indigo dye]] imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914.{{cn|date=August 2021}}
{{unreferenced section|date=April 2017}}

The [[French Army]] adopted dark blue indigo at the time of the [[French Revolution]], as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, [[Napoleon]] decided to restore the white coats because of shortages of [[indigo dye]] imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914.
=== Popular culture ===
In the ''[[Better Call Saul]]'' episode "[[Hero (Better Call Saul)|Hero]]", Howard Hamlin mentions that his law firm Hamlin Hamlin & McGill trademarked a colour called "Hamlindigo" whilst confronting Jimmy McGill over trademark infringement in a billboard advertisement he produced for his own legal services.


=== Spirituality ===
=== Spirituality ===
The [[spiritualist]] applications use [[#Electric indigo|electric indigo]], because the color is positioned between [[blue]] and [[Shades of violet#Electric violet|violet]] on the spectrum.<ref>Tansley, David W. ''Subtle Body: Essence and Shadow 1984 (Art and Cosmos Series--[[Jill Purce]], editor)</ref>
The [[Spiritualism (movement)|spiritualist]] applications use [[#Electric indigo|electric indigo]], because the color is positioned between [[blue]] and [[Shades of violet#Electric violet|violet]] on the spectrum.<ref>Tansley, David W. ''Subtle Body: Essence and Shadow'' 1984 (Art and Cosmos Series--[[Jill Purce]], editor)</ref>
* The color electric indigo is used in [[New Age]] philosophy to symbolically represent the sixth ''[[chakra]]'' (called ''Ajna''), which is said to include the [[third eye]]. This ''chakra'' is believed to be related to [[intuition]] and [[gnosis]] (spiritual knowledge).<ref>Stevens, Samantha. The Seven Rays: a Universal Guide to the Archangels. City: Insomniac Press, 2004. {{ISBN|1-894663-49-7}} pg. 24</ref><ref>Graham, Lanier F. (editor) ''The Rainbow Book'' Berkeley, California:1976 Shambala Publishing and The Fine Arts Museums of San Francisco (Handbook for the Summer 1976 exhibition ''The Rainbow Art Show'' which took place primarily at the [[De Young Museum]], but also at other museums) ''Indigo'' Pages 152–153 The color indigo is stated to represent intuition.</ref>
* The color electric indigo is used in [[New Age]] philosophy to symbolically represent the sixth ''[[chakra]]'' (called ''[[Ajna]]''), which is said to include the [[third eye]]. This ''chakra'' is believed to be related to [[intuition]] and [[gnosis]] (spiritual knowledge).<ref>Stevens, Samantha. The Seven Rays: a Universal Guide to the Archangels. City: Insomniac Press, 2004. {{ISBN|1-894663-49-7}} pg. 24</ref><ref>Graham, Lanier F. (editor) ''The Rainbow Book'' Berkeley, California:1976 Shambala Publishing and The Fine Arts Museums of San Francisco (Handbook for the Summer 1976 exhibition ''The Rainbow Art Show'' which took place primarily at the [[De Young Museum]], but also at other museums) ''Indigo'' Pages 152–153 The color indigo is stated to represent intuition.</ref>
* [[Alice A. Bailey]] used indigo as the "second ray", representing "Love-Wisdom", in her [[Seven Rays]] system classifying people into seven metaphysical [[psychological type]]s.<ref>{{cite book|last=Bailey|first=Alice A.|authorlink=Alice Bailey|title=The Seven Rays of Life|location=New York|year=1995|publisher=Lucis Publishing Company|isbn=978-0-85330-142-4}}</ref>
* [[Alice Bailey|Alice A. Bailey]] used indigo as the "second ray", representing "Love-Wisdom", in her [[Seven Rays]] system classifying people into seven metaphysical [[psychological type]]s.<ref>{{cite book|last=Bailey|first=Alice A.|author-link=Alice Bailey|title=The Seven Rays of Life|location=New York|year=1995|publisher=Lucis Publishing Company|isbn=978-0-85330-142-4}}</ref>
* [[Psychic]]s often associate indigo [[Aura (paranormal)|paranormal auras]] with an interest in [[religion]] or with intense [[spirituality]] and intuition. [[Indigo children]] are said to have predominantly indigo auras. People with indigo auras are said to favor occupations such as [[Computer analyst (disambiguation)|computer analyst]]<!-- dab link intentional -->, [[Animal husbandry|animal caretaker]], and [[Licensed professional counselor|counselor]].<ref>Oslie, Pamalie ''Life Colors: What the Colors in Your Aura Reveal'' Novato, California:2000--New World Library Indigo Auras: Pages 161–174</ref>
* [[Psychic]]s often associate indigo [[Aura (paranormal)|paranormal auras]] with an interest in [[religion]] or with intense [[spirituality]] and intuition. [[Indigo children]] are said to have predominantly indigo auras. People with indigo auras are said to favor occupations such as [[Computer analyst (disambiguation)|computer analyst]]<!-- dab link intentional -->, [[Animal husbandry|animal caretaker]], and [[Licensed professional counselor|counselor]].<ref>Oslie, Pamalie ''Life Colors: What the Colors in Your Aura Reveal'' Novato, California:2000--New World Library Indigo Auras: Pages 161–174</ref>
*In [[Wicca]], it represents emotion, fluidity, insight, and expressiveness. It is used to spiritually heal.<ref>{{Cite web|title=Magical Properties of Colors|url=https://wiccaliving.com/magical-properties-colors/|access-date=2021-01-28|website=Wicca Living|language=en-US|archive-date=22 January 2021|archive-url=https://web.archive.org/web/20210122023531/https://wiccaliving.com/magical-properties-colors/|url-status=live}}</ref>


== See also ==
== See also ==
* ''[[Baptisia]]'' (false indigo), a genus of flowering plants
* ''[[Baptisia]]'' (false indigo), a genus of flowering plants
* [[Indigo dye]], used in dyeing blue jeans their characteristic color
* [[Champaran Satyagraha]], the first pacific rebellion of [[Mahatma Gandhi]] against the [[British Raj]]
* ''[[Indigofera]]'', a genus of flowering plants
* ''[[Indigofera]]'', a genus of flowering plants
* [[Lists of colors]]
* [[Indiglo]], a brand name for a method of electroluminescence technology
* [[List of colors]]
* ''[[Persicaria tinctoria]]'', Japanese Indigo
* ''[[Persicaria tinctoria]]'', Japanese Indigo
* ''[[Red, White, and Black Make Blue: Indigo in the Fabric of Colonial South Carolina Life]]'' by [[Andrea Feeser]]
* [[Rainbow]], indigo is usually the sixth listed color of the rainbow
* [[Indigo dye]], used in dyeing blue jeans their characteristic color


{{-}}
== References ==
== References ==
{{Reflist}}
{{Reflist}}


== External links ==
== External links ==
*[https://archive.org/details/gri_c00033125012743312/page/n41/mode/2up 'Blues'] from ''Werner's Nomenclature of Colours'' (Edinburgh: William Blackwood, 1821) showing "Indigo" alongside other shades of blue
*{{Wikisource inline|Littell's Living Age/Volume 145/Issue 1869/Definition of the Color Indigo|An 1869 debate over whether "indigo" is an appropriate term for the spectral color.}}
*{{Wikisource inline|Littell's Living Age/Volume 145/Issue 1869/Definition of the Color Indigo|An 1869 debate over whether "indigo" is an appropriate term for the spectral color.}}


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{{Electromagnetic spectrum}}
{{Electromagnetic spectrum}}


[[Category:Tertiary colors]]
[[Category:Quaternary colors]]
[[Category:Optical spectrum]]
[[Category:Optical spectrum]]
[[Category:Rainbow]]
[[Category:Rainbow colors]]
[[Category:Shades of blue]]
[[Category:Shades of blue]]

Revision as of 17:12, 19 August 2024

Indigo
 
A piece of indigo plant dye from India,
about 6 cm (2.5 in) square
Spectral coordinates
Wavelength420–450[1] nm
Frequency~714–670 THz
About these coordinates     Color coordinates
Hex triplet#3800FF
sRGBB (r, g, b)(56, 0, 255)
HSV (h, s, v)(253°, 100%, 100%)
CIELChuv (L, C, h)(34, 132, 267°)
SourceCIECAM16 248:2022[2]
ISCC–NBS descriptorDark violet
B: Normalized to [0–255] (byte)

Indigo is a term used for a number of hues in the region of blue. The word comes from the ancient dye of the same name. The term "indigo" can refer to the color of the dye, various colors of fabric dyed with indigo dye, a spectral color, one of the seven colors of the rainbow as described by Newton, or a region on the color wheel, and can include various shades of blue, ultramarine, and green-blue. Since the web era, the term has also been used for various purple and violet hues identified as "indigo", based on use of the term "indigo" in HTML web page specifications.

The word "indigo" comes from the Latin word indicum, meaning "Indian", as the naturally based dye was originally exported to Europe from India.

The Early Modern English word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic color-naming system.[3]

The first known recorded use of indigo as a color name in English was in 1289.[4]

Isaac Newton regarded indigo as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between blue and violet; however, sources differ as to its actual position in the electromagnetic spectrum. Later scientists have concluded that what Newton called "blue" was what is now called cyan or blue-green; and what Newton called "indigo" was what is now called blue.

In the 1980s, programmers produced a somewhat arbitrary list of color names for the X Window computer operating system, resulting in the HTML and CSS specifications issued in the 1990s using the term "indigo" for a dark purple hue. This has resulted in violet and purple hues also being associated with the term "indigo" since that time.

Because of the Abney effect, pinpointing indigo to a specific hue value in the HSV color wheel is elusive, as a higher HSV saturation value shifts the hue towards blue. However, on the new CIECAM16 standard, the hues values around 290° may be thought of as indigo, depending on the observer.

History

Indigo as a dye

Extract of natural indigo applied to paper

Indigo dye is a blue color, obtained from several different types of plants. The indigo plant (Indigofera tinctoria) often called "true indigo" probably produces the best results, although several others are close in color: Japanese indigo (Polygonum tinctoria), Natal indigo (Indigofera arrecta), Guatemalan indigo (Indigofera suffruticosa), Chinese indigo (Persicaria tinctoria), and woad Isatis tinctoria.

Indigofera tinctoria and related species were cultivated in East Asia, Egypt, India, Bangladesh and Peru in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BC and comes from Huaca Prieta, in contemporary Peru.[5] Pliny the Elder mentions India as the source of the dye after which it was named.[6] It was imported from there in small quantities via the Silk Road.[7]

The Ancient Greek term for the dye was Ἰνδικὸν φάρμακον (indikon pharmakon, "Indian dye"), which, adopted to Latin as indicum (a second declension noun) or indico (oblique case) and via Portuguese, gave rise to the modern word indigo.[8]

Indigo extracted from woad

In early Europe the main source was from the woad plant Isatis tinctoria, also known as pastel.[9] For a long time, woad was the main source of blue dye in Europe. Woad was replaced by "true indigo", as trade routes opened up. Plant sources have now been largely replaced by synthetic dyes.

Spanish explorers discovered an American species of indigo and began to cultivate the product in Guatemala. The English and French subsequently began to encourage indigo cultivation in their colonies in the West Indies.[10]

In North America, indigo was introduced by Eliza Lucas into colonial South Carolina, where it became the colony's second-most important cash crop (after rice).[11] Before the Revolutionary War, indigo accounted for more than one-third of the value of exports from the American colonies.[12]

Isaac Newton's classification of indigo as a spectral color

Indigo is one of the colors on Newton's color wheel.

Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England,[13] supplanting the homegrown woad as source of blue dye. In a pivotal experiment in the history of optics, the young Newton shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this optical spectrum, Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."[14] He linked the seven prismatic colors to the seven notes of a western major scale,[15] as shown in his color wheel, with orange and indigo as the semitones. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:

Newton's observation of prismatic colors: Comparing this to a color image of the visible light spectrum shows that "blue" corresponds to cyan, while "indigo" corresponds to blue.

I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks.[16]

Traditional seven colors of the rainbow

Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonics "Richard of York gave battle in vain" and Roy G. Biv. James Clerk Maxwell and Hermann von Helmholtz accepted indigo as an appropriate name for the color flanking violet in the spectrum.[17]

Later scientists concluded that Newton named the colors differently from current usage.[18][19] According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name blue-green or cyan."[20] If this is true, Newton's seven spectral colors would have been:

Red: Orange: Yellow: Green: Blue:  Indigo: Violet: 

The human eye does not readily differentiate hues in the wavelengths between what are now called blue and violet. If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to Isaac Asimov, "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes, it seems merely deep blue."[21]

1800s

In 1821, Abraham Werner published Werner's Nomenclature of Colours, where indigo, called indigo blue, is classified as a blue hue, and not listed among the violet hues. He writes that the color is composed of "Berlin blue, a little black, and a small portion of apple green," and indicating it is the color of blue copper ore, with Berlin blue being described as the color of a blue jay's wing, a hepatica flower, or a blue sapphire.[22]

According to an article, Definition of the Color Indigo published in Nature magazine in the late 1800s, Newton's use of the term "indigo" referred to a spectral color between blue and violet. However, the article states that Wilhelm von Bezold, in his treatise on color, disagreed with Newton's use of the term, on the basis that the pigment indigo was a darker hue than the spectral color; and furthermore, Professor Ogden Rood points out that indigo pigment corresponds to the cyan-blue region of the spectrum, lying between blue and green, although darker in hue. Rood considers that artificial ultramarine pigment is closer to the point of the spectrum described as "indigo", and proposed renaming that spectral point as "ultramarine". The article goes on to state that comparison of the pigments, both dry and wet, with Maxwell's discs and with the spectrum, that indigo is almost identical to Prussian blue, stating that it "certainly does not lie on the violet side of 'blue.'" When scraped, a lump of indigo pigment appears more violet, and if powdered or dissolved, becomes greenish.[23]

Modern spectral classification

Several modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers,[1][24][25] which lies on the short-wave side of color wheel (RGB) blue, towards (spectral) violet.

The correspondence of this definition with colors of actual indigo dyes, though, is disputed. Optical scientists Hardy and Perrin list indigo as between 445[26] and 464 nm wavelength,[27] which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards azure.

Other modern color scientists, such as Bohren and Clothiaux (2006), and J.W.G. Hunt (1980), divide the spectrum between violet and blue at about 450 nm, with no hue specifically named indigo.[28][29]

Web era

Origin of "Indigo" as a name for purple in web pages

Towards the end of the 20th century, purple colors also became referred to as "indigo". In the 1980s, computer programmers Jim Gettys, Paul Ravelling, John C. Thomas and Jim Fulton produced a list of colors for the X Window Operating System. The color identified as "indigo" was not the color indigo (as generally understood at the time), but was actually a dark purple hue; the programmers assigned it the hex code #4B0082 . This collection of color names was somewhat arbitrary: Thomas used a box of 72 Crayola crayons as a standard, whereas Ravelling used color swabs from the now-defunct Sinclair Paints company, resulting in the color list for version X11 of the operating system containing fanciful color names such as "papaya whip", "blanched almond" and "peach puff". The database was also criticised for its many inconsistencies, such as "dark gray" being lighter than "gray", and for the color distribution being uneven, tending towards reds and greens at the expense of blues.[30]

In the 1990s, this list which came with version X11 became the basis of the HTML and CSS color rendition used in websites and web design. This resulted in the name "Indigo" being associated with purple and violet hues in web page design and graphic design. Physics author John Spacey writes on the website Simplicable that the X11 programmers did not have any background in color theory, and that as these names are used by web designers and graphic designers, the name indigo has since that time been strongly associated with purple or violet. Spacey writes, "As such, a few programmers accidentally repurposed a color name that was known to civilisations for thousands of years."[31]

Crayola crayon colors

The Crayola company released an indigo crayon in 1999, with the Crayola website using the hex code #4F49C6 to approximate the crayon color. The 2001 iron indigo crayon is portrayed using hex code #184FA1 , the 2004 indigo crayon color uses #5D76CB , the 2019 iridescent indigo uses #3C32CD .

Distinction among tones of indigo

Like many other colors (orange, rose, and violet are the best-known), indigo gets its name from an object in the natural world—the plant named indigo once used for dyeing cloth (see also Indigo dye).

The color pigment indigo is equivalent to the web color indigo and approximates the color indigo that is usually reproduced in pigments and colored pencils.

The color of indigo dye is a different color from either spectrum indigo or pigment indigo. This is the actual color of the dye. A vat full of this dye is a darker color, approximating the web color midnight blue.

The color "electric indigo" is a bright and saturated color between the traditional indigo and violet. This is the brightest color indigo that can be approximated on a computer screen; it is a color located between the (primary) blue and the color violet of the RGB color wheel.

The web color blue violet or deep indigo is a tone of indigo brighter than pigment indigo, but not as bright as electric indigo.

Listed below are several indigo hues, some of which have included the word "indigo", with the adoption of HTML color names in the World Wide Web era.

Indigo dye color

Indigo Dye
 
About these coordinates     Color coordinates
Hex triplet#00416A
sRGBB (r, g, b)(0, 65, 106)
HSV (h, s, v)(203°, 100%, 42%)
CIELChuv (L, C, h)(26, 40, 246°)
Source[1]
ISCC–NBS descriptorDeep blue
B: Normalized to [0–255] (byte)

Indigo dye is a greenish dark blue color, obtained from either the leaves of the tropical Indigo plant (Indigofera), or from woad (Isatis tinctoria), or the Chinese indigo (Persicaria tinctoria). Many societies make use of the Indigofera plant for producing different shades of blue. Cloth that is repeatedly boiled in an indigo dye bath-solution (boiled and left to dry, boiled and left to dry, etc.), the blue pigment becomes darker on the cloth. After dyeing, the cloth is hung in the open air to dry.

A Native American woman described the process used by the Cherokee Indians when extracting the dye:

We raised our indigo which we cut in the morning while the dew was still on it; then we put it in a tub and soaked it overnight, and the next day we foamed it up by beating it with a gourd. We let it stand overnight again, and the next day rubbed tallow on our hands to kill the foam. Afterwards, we poured the water off, and the sediment left in the bottom we would pour into a pitcher or crock to let it get dry, and then we would put it into a poke made of cloth (i.e. sack made of coarse cloth) and then when we wanted any of it to dye [there]with, we would take the dry indigo.[32][33]

In Sa Pa, Vietnam, the tropical Indigo (Indigo tinctoria) leaves are harvested and, while still fresh, placed inside a tub of room-temperature to lukewarm water where they are left to sit for 3 to 4 days and allowed to ferment, until the water turns green. Afterwards, crushed limestone (pickling lime) is added to the water, at which time the water with the leaves are vigorously agitated for 15 to 20 minutes, until the water turns blue. The blue pigment settles as sediment at the bottom of the tub. The sediment is scooped out and stored. When dyeing cloth, the pigment is then boiled in a vat of water; the cloth (usually made from yarns of hemp) is inserted into the vat for absorbing the dye. After hanging out to dry, the boiling process is repeated as often as needed to produce a darker color.

Indigo (color wheel)

Indigo (color wheel)
 
About these coordinates     Color coordinates
Hex triplet#4000FF[34]
sRGBB (r, g, b)(64, 0, 255)
HSV (h, s, v)(255°, 100%, 100%)
CIELChuv (L, C, h)(35, 133, 268°)
Source[Unsourced]
ISCC–NBS descriptorVivid purplish blue
B: Normalized to [0–255] (byte)

In a RGB color space, "Indigo(color wheel)" is composed of 25.1% red, 0% green and 100% blue. Whereas in a CMYK color space, it is composed of 74.9% cyan, 100% magenta, 0% yellow and 0% black. It has a hue angle of 255.1 degrees, a saturation of 100% and a lightness of 50%. Indigo(color wheel) could be obtained by blending violet with blue.

Electric indigo

Electric Indigo
 
About these coordinates     Color coordinates
Hex triplet#6F00FF[34]
sRGBB (r, g, b)(111, 0, 255)
HSV (h, s, v)(266°, 100%, 100%)
CIELChuv (L, C, h)(39, 134, 273°)
Source[Unsourced]
ISCC–NBS descriptorVivid purplish blue
B: Normalized to [0–255] (byte)

"Electric indigo" is brighter than the pigment indigo reproduced above. When plotted on the CIE chromaticity diagram, this color is at 435 nanometers, in the middle of the portion of the spectrum traditionally considered indigo, i.e., between 450 and 420 nanometers. This color is only an approximation of spectral indigo, since actual spectral colors are outside the gamut of the sRGB color system.

Deep indigo (web color blue-violet)

Blue-Violet
 
About these coordinates     Color coordinates
Hex triplet#8A2BE2
sRGBB (r, g, b)(138, 43, 226)
HSV (h, s, v)(271°, 81%, 89%)
CIELChuv (L, C, h)(42, 115, 279°)
SourceX11
ISCC–NBS descriptorVivid violet
B: Normalized to [0–255] (byte)

At right is displayed the web color "blue-violet", a color intermediate in brightness between electric indigo and pigment indigo. It is also known as "deep indigo".

Web color indigo

Web color Indigo
 
About these coordinates     Color coordinates
Hex triplet#4B0082
sRGBB (r, g, b)(75, 0, 130)
HSV (h, s, v)(275°, 100%, 51%)
CIELChuv (L, C, h)(20, 62, 279°)
Source[2]
ISCC–NBS descriptorDark violet
B: Normalized to [0–255] (byte)
Web safe Indigo
 
About these coordinates     Color coordinates
Hex triplet#330099
sRGBB (r, g, b)(51, 0, 153)
HSV (h, s, v)(260°, 100%, 60%)
CIELChuv (L, C, h)(20, 73, 270°)
Source[3]
B: Normalized to [0–255] (byte)

The color box on the right displays the web color indigo, the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that is possible to reproduce on a computer screen. Its hue is closer to violet than to indigo dye for which the color is named. Pigment indigo can be obtained by mixing 55% pigment cyan with about 45% pigment magenta.

Compare the subtractive colors to the additive colors in the two primary color charts in the article on primary colors to see the distinction between electric colors as reproducible from light on a computer screen (additive colors) and the pigment colors reproducible with pigments (subtractive colors); the additive colors are significantly brighter because they are produced from light instead of pigment.

Web color indigo represents the way the color indigo was always reproduced in pigments, paints, or colored pencils in the 1950s. By the 1970s, because of the advent of psychedelic art, artists became accustomed to brighter pigments. Pigments called "bright indigo" or "bright blue-violet" (the pigment equivalent of the electric indigo reproduced in the section above) became available in artists' pigments and colored pencils.

Tropical indigo

Tropical Indigo
 
About these coordinates     Color coordinates
Hex triplet#9683EC
sRGBB (r, g, b)(150, 131, 236)
HSV (h, s, v)(251°, 44%, 93%)
CIELChuv (L, C, h)(61, 84, 271°)
SourceGallego and Sanz[35]
ISCC–NBS descriptorVivid violet
B: Normalized to [0–255] (byte)

'Tropical Indigo' is the color that is called añil in the Guía de coloraciones (Guide to colorations) by Rosa Gallego and Juan Carlos Sanz, a color dictionary published in 2005 that is widely popular in the Hispanophone realm.

Imperial blue

Imperial Blue
 
About these coordinates     Color coordinates
Hex triplet#002395
sRGBB (r, g, b)(0, 35, 149)
HSV (h, s, v)(226°, 100%, 58%)
CIELChuv (L, C, h)(21, 72, 263°)
Source[Unsourced]
ISCC–NBS descriptorVivid blue
B: Normalized to [0–255] (byte)

In nature

Birds

Indigo bunting
Indigo bunting

Fungi

An upturned Lactarius indigo mushroom
An upturned Lactarius indigo mushroom

Snakes

Eastern indigo snake
Eastern indigo snake
  • The eastern indigo snake, Drymarchon couperi, of the southeastern United States, is a dark blue/black.

In culture

Business

  • IndiGo is an Indian budget airline that uses an indigo logo and operates only Airbus A320s.
  • Indigo Books and Music uses an indigo logo and has sometimes referred to the color as "blue" in advertising.[36][37]
  • The GameCube was initially released in 2 color variants, including one bearing the title of 'Indigo', with the main console and controllers in that color.

Computer graphics

  • Electric indigo is sometimes used as a glow color for computer graphics lighting, possibly because it seems to change color from indigo to lavender when blended with white.

Dyes

Indigo is created in potholes carved in pumice "tufgrond" in Karoland, Sumatra.
  • Indigo dye was used to dye denim, giving the original 'blue jeans' their distinctive colour.
  • The original Postal Worker uniform contained indigo dye, partly due to the dye not running when wet. [38]
  • Guatemala, as of 1778, was considered one of the world's foremost providers of indigo.[39]
  • In Mexico, indigo is known as añil.[40] After silver, and cochineal to produce red, añil was the most important product exported by historical Mexico.[41]
  • The use of añil is survived in the Philippines, particularly in the Visayas and Mindanao. The powder dye is mixed with vinegar to be applied to the cheek of a person suffering from mumps.[citation needed]

Food

  • Scientists discovered in 2008 that when a banana becomes ripe, it glows bright indigo under a black light. Some insects, as well as birds, see into the ultraviolet, because they are tetrachromats and can use this information to tell when a banana is ready to eat. The glow is the result of a chemical created as the green chlorophyll in the peel breaks down.[42]

Literature

Marina Warner's novel Indigo (1992) is a retelling of Shakespeare's The Tempest and features the production of indigo dye by Sycorax.

Military

The French Army adopted dark blue indigo at the time of the French Revolution, as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, Napoleon decided to restore the white coats because of shortages of indigo dye imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914.[citation needed]

In the Better Call Saul episode "Hero", Howard Hamlin mentions that his law firm Hamlin Hamlin & McGill trademarked a colour called "Hamlindigo" whilst confronting Jimmy McGill over trademark infringement in a billboard advertisement he produced for his own legal services.

Spirituality

The spiritualist applications use electric indigo, because the color is positioned between blue and violet on the spectrum.[43]

See also

References

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  2. ^ CIECAM16 248:2022 Archived 20 April 2024 at the Wayback Machine. J = 41, C = 74, h = 289
  3. ^ Ottenheimer, Harriet Joseph (2009). The anthropology of language: an introduction to linguistic anthropology (2nd ed.). Belmont, CA: Wadsworth. p. 29. ISBN 978-0-495-50884-7.
  4. ^ Maerz and Paul A Dictionary of Color New York:1930 McGraw-Hill Page 197; Color Sample of Indigo: Page 117 Plate 47 Color Sample E10
  5. ^ Splitstoser, Jeffrey C.; Dillehay, Tom D.; Wouters, Jan; Claro, Ana (September 2016). "Early pre-Hispanic use of indigo blue in Peru". Science Advances. 2 (9): e1501623. Bibcode:2016SciA....2E1623S. doi:10.1126/sciadv.1501623. PMC 5023320. PMID 27652337.
  6. ^ "Night of the Indigo". harappa.com. Archived from the original on 14 January 2019. Retrieved 20 May 2016.
  7. ^ Clark, Robin J. H.; Cooksey, Christopher J.; Daniels, Marcus A. M.; Withnall, Robert (1993). "Indigo, woad, and Tyrian Purple: important vat dyes from antiquity to the present". Endeavour. 17 (4): 191–199. doi:10.1016/0160-9327(93)90062-8.
  8. ^ Ἰνδικός Archived 29 January 2021 at the Wayback Machine in Henry George Liddell. Robert Scott. A Greek-English Lexicon. revised and augmented throughout by. Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford. Clarendon Press. 1940; English indigo since the 17th century, changed from 16th-century indico.
  9. ^ "Getting the blues: the pastel trade in southwest France". Life on La Lune. 22 May 2011. Retrieved 23 February 2018.
  10. ^ Pritchard, James (2004). In Search of Empire: The French in the Americas, 1670–1730. Cambridge: Cambridge University Press. p. 127.
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  12. ^ "Eliza Lucas Pinckney" Archived 21 November 2008 at the Wayback Machine, Biographies, National Women's History Museum, 2007, accessed 7 December 2008.
  13. ^ Allen, O.N. Allen & Ethel K. (1981). The Leguminosae: a source book of characteristics, uses, and nodulation (null ed.). Madison, Wisc.: University of Wisconsin Press. p. 343. ISBN 978-0-299-08400-4.
  14. ^ Newton's draft of A Theory Concerning Light and Colors Archived 21 July 2011 at the Wayback Machine on newtonproject.sussex.ac.uk
  15. ^ "SHiPS Resource Center || Newton's Colors". Archived from the original on 29 September 2014. Retrieved 16 October 2010.
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  20. ^ Waldman, Gary (1983). Introduction to light : the physics of light, vision, and color (2002 Dover revised ed.). Mineola: Dover Publications. p. 193. ISBN 978-0-486-42118-6.
  21. ^ Asimov, Isaac (1975). Eyes on the universe : a history of the telescope. Boston: Houghton Mifflin. p. 59. ISBN 978-0-395-20716-1.
  22. ^ Werner, Abraham (1821). Werner's Nomenclature of Colours. London. p. 41. Retrieved 30 October 2023.
  23. ^ "Definition of the Color Indigo". Littel's Living Age. 145 (1869). 1880. Archived from the original on 30 October 2023. Retrieved 30 October 2023. Newton denoted by the name of "indigo" the tint of the spectrum lying between "blue" and "violet." Von Bezold, in his work on color, rejects the term, justifying his objection by observing that the pigment indigo is a much darker hue than the spectrum tint. Prof. O. N. Rood, who follows Von Bezold in rejecting the term, brings forward the further objection that the tint of the pigment indigo more nearly corresponds in hue (though it is darker) with the cyan-blue region lying between green and blue. By comparing the tints of indigo pigment, both dry and wet, with the spectrum, and by means of Maxwell's disks, it appears that the hue of indigo is almost identical with that of Prussian blue, and certainly does not lie on the violet side of "blue." Indigo in the dry lump, if scraped, has, however, a more violet tint; but if fractured or powdered, or dissolved, its tint is distinctly greenish. Prof. Rood considers that artificial ultramarine corresponds much more nearly to the true tint of the spectrum at the point usually termed "indigo," and he therefore proposes to substitute the term "ultramarine" in its place, the color of the artificial pigment being thereby intended.
  24. ^ Group, The HURIS. "Spectrum of Electromagnetic Radiation ( EMR )". www.huris.com. Archived from the original on 28 January 2020. Retrieved 14 June 2013. {{cite web}}: |last= has generic name (help)
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  26. ^ "What Wavelength Goes with a Color?". Archived from the original on 24 November 2016. Retrieved 24 November 2016.
  27. ^ Arthur C. Hardy and Fred H. Perrin. The Principles of Optics. McGraw-Hill Book Co., Inc., New York. 1932.
  28. ^ Craig F. Bohren and Eugene E. Clothiaux (2006). Fundamentals of Atmospheric Radiation. Wiley-VCH. ISBN 978-3-527-40503-9.[permanent dead link]
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  30. ^ Tveten, Julianne (11 October 2015). ""Tomato" versus "#FF6347"—the tragicomic history of CSS color names". Ars Technica. Archived from the original on 29 October 2023. Retrieved 11 October 2015.
  31. ^ Spacey, John (19 June 2020). "19 Types of Indigo". Simplicable. Archived from the original on 5 June 2023. Retrieved 30 October 2023. In 1986 some programmers created a list of color names for a unix system known as X11. Having no background in color theory, they placed indigo as a dark purple. This list was later used by HTML and CSS standards that remain in place to this day. These standards are used by millions of designers and digital artists such that the color name indigo is now strongly associated with dark purple or violet. As such, a few programmers accidentally repurposed a color name that was known to civilizations for thousands of years. ...Note the difference between Web Indigo and Indigo. This standard color name is completely detached from the traditional color. This misrepresentation resulted from the random selection by a programmer working on an operating system in 1986.
  32. ^ Knight, Oliver (1956–57), "History of the Cherokees, 1830–1846", Chronicles of Oklahoma, Oklahoma City: Oklahoma Historical Society, p. 164, OCLC 647927893
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  34. ^ a b "What Color is Indigo?". 16 December 2014. Archived from the original on 26 December 2015.
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  36. ^ "It's New and It's Blue" (Indigo advertisement), The Globe and Mail, Toronto, 1 October 1999, p. A3
  37. ^ "Indigo Bookstore had a 'Think Blue' campaign back in 1999" according to: "Think Blue 2008: a Before and After Tale of Silly Turf Battles and Redemptive Communication". Archived from the original on 10 May 2012. Retrieved 4 February 2013.[better source needed]
  38. ^ Hejna, Jamie (19 February 2023). "What Color is Indigo?". Freelance Faucet. Archived from the original on 19 February 2023. Retrieved 19 February 2023.
  39. ^ Kitchin, Thomas (1778). The Present State of the West-Indies: Containing an Accurate Description of What Parts Are Possessed by the Several Powers in Europe. London: R. Baldwin. p. 30. Archived from the original on 19 October 2013. Retrieved 30 August 2013.
  40. ^ Gallego, Rosa; Sanz, Juan Carlos (2001). Diccionario Akal del color. Akal. ISBN 978-84-460-1083-8.
  41. ^ Article „añil“ in: Enciclopedia de México, vol 1, Mexiko-City: Secretaría de Educacion Pública, 1987
  42. ^ Zurer, Rachel. "Three Smart Things About Banana Peels". Wired. Archived from the original on February 28, 2020. Retrieved February 28, 2020.
  43. ^ Tansley, David W. Subtle Body: Essence and Shadow 1984 (Art and Cosmos Series--Jill Purce, editor)
  44. ^ Stevens, Samantha. The Seven Rays: a Universal Guide to the Archangels. City: Insomniac Press, 2004. ISBN 1-894663-49-7 pg. 24
  45. ^ Graham, Lanier F. (editor) The Rainbow Book Berkeley, California:1976 Shambala Publishing and The Fine Arts Museums of San Francisco (Handbook for the Summer 1976 exhibition The Rainbow Art Show which took place primarily at the De Young Museum, but also at other museums) Indigo Pages 152–153 The color indigo is stated to represent intuition.
  46. ^ Bailey, Alice A. (1995). The Seven Rays of Life. New York: Lucis Publishing Company. ISBN 978-0-85330-142-4.
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