olive brown vs brownness

olive brown and brownness both are nouns.

Word	Noun	Adjective	Verb	Adverb
olive brown	Yes	No	No	No
brownness	Yes	No	No	No

As nouns, brownness is a hypernym of olive brown; that is, brownness is a word with a broader meaning than olive brown:

Other hypernyms of olive brown include brown.

olive brown (noun)	brownness (noun)
a shade of brown tinged with green	an orange of low brightness and saturation