sweet violet vs violet

sweet violet and violet both are nouns.

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

As nouns, violet is a hypernym of sweet violet; that is, violet is a word with a broader meaning than sweet violet:

sweet violet: European violet typically having purple to white flowers; widely naturalized
violet: any of numerous low-growing violas with small flowers

sweet violet (noun)	violet (noun)
European violet typically having purple to white flowers; widely naturalized	a variable color that lies beyond blue in the spectrum
	any of numerous low-growing violas with small flowers

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