violet vs indigo

violet and indigo both are nouns.

Word	Noun	Adjective	Verb	Adverb
violet	Yes	Yes	No	No
indigo	Yes	Yes	No	No

As nouns, indigo is a hyponym of violet; that is, indigo is a word with a more specific, narrower meaning than violet:

Other hyponyms of violet include bishop's purple, bishop's violet.

violet (noun)	indigo (noun)
a variable color that lies beyond blue in the spectrum	a blue-violet color
any of numerous low-growing violas with small flowers	deciduous subshrub of southeastern Asia having pinnate leaves and clusters of red or purple flowers; a source of indigo dye
	a blue dye obtained from plants or made synthetically

violet (adjective)	indigo (adjective)
of a color intermediate between red and blue	having a color between blue and violet