green tea vs tea

green tea and tea both are nouns.

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

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

Other hypernyms of green tea include tea leaf.

green tea (noun)	tea (noun)
tea leaves that have been steamed and dried without fermenting	a beverage made by steeping tea leaves in water
	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