greens vs veggie

greens and veggie both are nouns.

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

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

greens: any of various leafy plants or their leaves and stems eaten as vegetables
veggie: edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant

Other hypernyms of greens include veg, vegetable.

greens (noun)	veggie (noun)
any of various leafy plants or their leaves and stems eaten as vegetables	edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant