tea vs green tea

tea and green tea both are nouns.

Word	Noun	Adjective	Verb	Adverb
tea	Yes	No	No	No
green tea	Yes	No	No	No

As nouns, green tea is a hyponym of tea; that is, green tea is a word with a more specific, narrower meaning than tea:

Other hyponyms of tea include tea bag, black tea, oolong.

tea (noun)	green tea (noun)
a beverage made by steeping tea leaves in water	tea leaves that have been steamed and dried without fermenting
a light midafternoon meal of tea and sandwiches or cakes
dried leaves of the tea shrub; used to make tea
a tropical evergreen shrub or small tree extensively cultivated in e.g. China and Japan and India; source of tea leaves