reddish blue vs indigo

reddish blue and indigo both are nouns.

Word	Noun	Adjective	Verb	Adverb
reddish blue	Yes	No	No	No
indigo	Yes	Yes	No	No

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

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

reddish blue (noun)	indigo (noun)
a variable color that lies beyond blue in the spectrum	a blue-violet color
	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

reddish blue (adjective)	indigo (adjective)
	having a color between blue and violet