turnip greens vs greens

turnip greens and greens both are nouns.

Word	Noun	Adjective	Verb	Adverb
turnip greens	Yes	No	No	No
greens	Yes	No	No	No

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

turnip greens: tender leaves of young white turnips
greens: any of various leafy plants or their leaves and stems eaten as vegetables

Other hypernyms of turnip greens include green, leafy vegetable.

turnip greens (noun)	greens (noun)
tender leaves of young white turnips	any of various leafy plants or their leaves and stems eaten as vegetables