string bean vs green bean

string bean and green bean both are nouns.

Word	Noun	Adjective	Verb	Adverb
string bean	Yes	No	No	No
green bean	Yes	No	No	No

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

string bean (noun)	green bean (noun)
green beans with strings that must be removed	immature bean pod eaten as a vegetable
	seed of the mung bean plant used for food; food from the seed of the mung bean plant
	a common bean plant cultivated for its slender green edible pods
	seed of the mung bean plant; used for food; seed of the mung bean plant