spinach vs greens

spinach and greens both are nouns.

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

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

spinach: dark green leaves; eaten cooked or raw in salads
greens: any of various leafy plants or their leaves and stems eaten as vegetables

Other hypernyms of spinach include green, leafy vegetable.

spinach (noun)	greens (noun)
dark green leaves; eaten cooked or raw in salads	any of various leafy plants or their leaves and stems eaten as vegetables
southwestern Asian plant widely cultivated for its succulent edible dark green leaves