Indigo is a color that 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. Indigo is a deep and bright color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine.
The first known recorded use of indigo as a color name in English was in 1289
Crayola swapped out a color in 1999 in their core lineup, that didn’t get any publicity but is referenced on their web site chronology. Thistle was replaced by Indigo. Indigo was a brand new color name and true color.