brown vs brownish

brown is a noun but brownish is not a noun.

Word	Noun	Adjective	Verb	Adverb
brown	Yes	Yes	Yes	No
brownish	No	Yes	No	No

As adjectives, brown and brownish are synonyms defined as:

Other synonyms of brown include chocolate-brown, dark-brown.

brown (noun)	brownish (noun)
an orange of low brightness and saturation

brown (adjective)	brownish (adjective)
(of skin) deeply suntanned	of a color similar to that of wood or earth
of a color similar to that of wood or earth

brown (verb)	brownish (verb)
fry in a pan until it changes color
make brown in color