beet green vs greens

beet green and greens both are nouns.

Word	Noun	Adjective	Verb	Adverb
beet green	Yes	No	No	No
greens	Yes	No	No	No

As nouns, greens is a hypernym of beet green; that is, greens is a word with a broader meaning than beet green:

beet green: young leaves of the beetroot
greens: any of various leafy plants or their leaves and stems eaten as vegetables

Other hypernyms of beet green include green, leafy vegetable.

beet green (noun)	greens (noun)
young leaves of the beetroot	any of various leafy plants or their leaves and stems eaten as vegetables