grape juice vs fruit juice

grape juice and fruit juice both are nouns.

Word	Noun	Adjective	Verb	Adverb
grape juice	Yes	No	No	No
fruit juice	Yes	No	No	No

As nouns, fruit juice is a hypernym of grape juice; that is, fruit juice is a word with a broader meaning than grape juice:

Other hypernyms of grape juice include fruit crush.

grape juice (noun)	fruit juice (noun)
the juice of grapes	drink produced by squeezing or crushing fruit