indigo, one.

track one of c o l o r s describing light with sound : 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. Although traditionally considered one of seven major spectral colors, its actual position in the electromagnetic spectrum is controversial. 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.