white paper vs written report

white paper and written report both are nouns.

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

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

white paper: a government report; bound in white
written report: a written document describing the findings of some individual or group

Other hypernyms of white paper include report, study.

white paper (noun)	written report (noun)
a government report; bound in white	a written document describing the findings of some individual or group