green paper vs written report

green paper and written report both are nouns.

Word	Noun	Adjective	Verb	Adverb
green paper	Yes	No	No	No
written report	Yes	No	No	No

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

green paper: a preliminary report of government proposals that is published in order to stimulate discussion
written report: a written document describing the findings of some individual or group

Other hypernyms of green paper include report, study.

green paper (noun)	written report (noun)
a preliminary report of government proposals that is published in order to stimulate discussion	a written document describing the findings of some individual or group