Editing Indigo (section)

== History ==

===Indigo as a dye===
{{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]]''.

''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>

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>

[[File:Isatis tinctoria 003.JPG|thumb|upright=0.8|Indigo extracted from woad]]
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>

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's classification of indigo as a spectral color ===
[[File:Newton's colour circle.png|thumb|class=skin-invert-image|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 [[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:
[[File:Newton prismatic colours.JPG|thumb|class=skin-invert-image|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|upright=0.86|The 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>
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.{{citation needed|date=March 2025}} 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>

===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 "[[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>

===Modern spectral classification===
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 is depicted by #5D76CB {{Colorsample|#5D76CB|1}}, and the 2019 iridescent indigo is portrayed by #3C32CD {{Colorsample|#3C32CD|1}}.