violet vs viola

violet and viola both are nouns.

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

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

violet (noun)	viola (noun)
a variable color that lies beyond blue in the spectrum	any of the numerous plants of the genus Viola
any of numerous low-growing violas with small flowers	a bowed stringed instrument slightly larger than a violin, tuned a fifth lower

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