go-ahead vs green light

go-ahead and green light both are nouns.

Word	Noun	Adjective	Verb	Adverb
go-ahead	Yes	No	No	No
green light	Yes	No	No	No

As nouns, go-ahead and green light are synonyms defined as:

go-ahead (noun)	green light (noun)
readiness to embark on bold new ventures	a signal to proceed
a signal to proceed	permission to proceed with a project or to take action