An Etymological Dictionary of Astronomy and Astrophysics

A color between blue and violet.

Etymology (EN): From Sp. indico, Portuguese endego, and Du. (via Portuguese) indigo, from L. indicum “indigo,” from Gk. indikon “blue dye from India,” literally “Indian (substance),” neuter of indikos “Indian,” → Indus.

Etymology (PE): Nil “indigo color; the indigo plant;” Mid.Pers. nil; cf. Skt. nīlī, nīlā “the indigo plant; indigo color.”