Indigo is traditionally regarded as a color on the visible spectrum, as well as one of the seven colors of the rainbow: the color between blue and violet. It is counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonic Roy G. Biv.
The color indigo was named after the indigo dye derived from the plant Indigofera tinctoria and related species. Species of Indigofera were cultivated in India, East Asia and Egypt in antiquity. The same indigo dye is contained in the woad plant, Isatis tinctoria, which was the main source of blue dye in Europe for many years. Woad was replaced by true indigo as trade routes opened up, and both are now largely replaced by synthetic dyes. Isaac Newton introduced indigo as one of the seven colors in his spectrum. 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, replacing homegrown woad as the source of blue dye. Indigofera tinctoria
, also called true indigo, is a species of plant from the bean family that was one of the original sources of indigo dye. Today most indigo dye is synthetic, but natural dye from I. tinctoria is still available, marketed as a natural colorant. True indigo is a shrub one to two meters high. It may be an annual, biennial, or perennial, depending on the climate in which it is grown. It has light green pinnate leaves and sheafs of pink or violet flowers. The plant is a legume, so it is rotated into fields to improve the soil in much the same way as other legume crops such as alfalfa and beans. Because I. tinctoria was widely introduced for dye production in tropical regions around the world and escaped from cultivation, it is now considered an agricultural and environmental weed, with the potential to threaten native ecosystems
Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once very rare. A large percentage of indigo dye produced today, several thousand tons each year, is synthetic. Indigo is the blue that we often associate with blue jeans.I. tinctoria
has also been used as a hair dye and there have been concerns of its possible harm to human health. In April 2004, the Scientific Committee on Cosmetic Products and Non-Food Products identified the need for a complete safety dossier of the species, and a review of its potential toxicity was conducted by the EU Scientific Committee on Consumer Safety which concluded in 2012 that further research is required, as currently there is insufficient data. 
Clinical trials in Taiwan showed that an ointment made from another species called indigo naturalis (Qing Dai), a dark blue plant-based powder used in traditional Chinese medicine, appears effective in treating plaque-type psoriasis. 
To extract indigo dye from its plant source, the leaves were soaked in water and fermented to convert the glycoside indican present in the plant to the blue dye indigotin. The precipitate from the fermented leaf solution was mixed with a strong base such as lye, pressed into cakes, dried, and powdered. The powder was then mixed with various other substances to produce different shades of blue and purple. Because of its high value as a trading commodity, indigo was often referred to as blue gold.  Indigo was the foundation of centuries-old textile traditions throughout West Africa. From the Tuareg nomads of the Sahara to Cameroon, clothes dyed with indigo signified wealth.Indigo carmine
is an indigo derivative which is also used as a colorant. About 20 million kg are produced annually, again mainly for blue jeans. It is also used as a food colorant, and is listed in the United States as FD&C Blue No. 2. FD&C Blue No. 2 is obtained by heating indigo (or indigo paste) in the presence of sulfuric acid. 
It is important for formulators to note that several plant species of indigo are toxic to humans and animals and should be avoided. I. spicata is recognized as a teratogen due to the presence of indospicine. Indospicine also is hepatotoxic. In animals, it causes cleft palate and embryo lethality. In addition, I. endacaphylla (creeping indigo) has been responsible for livestock poisonings and deaths. With the lack of reputable safety data and concerns about possible toxicity, true indigo should be used in cosmetic applications with caution.References:
- 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.
- SCCS, 2012. Opinion on Indigofera tinctoria, Scientific Committee on Consumer Safety, 16th Plenary Meeting, 18 September 2012. http://ec.europa.eu/health/scientific_committees/consumer_safety/docs/sccs_o_110.pdf
- Lin et al. Clinical Assessment of Patients With Recalcitrant Psoriasis in a Randomized, Observer-Blind, Vehicle-Controlled Trial Using Indigo Naturalis. Archives of Dermatology, 2008; 144 (11): 1457 DOI:10.1001/archderm.144.11.1457
- "History of Indigo & Indigo Dyeing". wildcolours.co.uk. Wild Colours and natural Dyes. Retrieved 30 December 2015.
- FDA Certified Colorants: http://www.ecfr.gov/cgi-bin/retrieveECFR?gp=&SID=ac0d565b513505082fd33f…