Indigo is a color named after the blue dye derived from the plant Indigofera tinctoria and related species. The color is placed on the electromagnetic spectrum between about 420 and 450 nm in wavelength, placing it between blue and violet. Although traditionally considered one of seven colors of the rainbow or the optical spectrum, modern color scientists do not usually recognize indigo as a separate division and generally classify wavelengths shorter than about 450 nm as violet. Optical scientists Hardy and Perrin list indigo as between 446 and 464 nm wavelength.
The first recorded use of indigo as a color name in English was in 1289.
Read more about Indigo: History, Classification As A Spectral Color, Distinction Between Four Major Tones of Indigo, In Nature