brown vs color in

brown is a noun but color in is not a noun.

Word	Noun	Adjective	Verb	Adverb
brown	Yes	Yes	Yes	No
color in	No	No	Yes	No

As verbs, color in is a hypernym of brown; that is, color in is a word with a broader meaning than brown:

Other hypernyms of brown include color, colorise, colorize, colour, colour in, colourise, colourize.

brown (noun)	color in (noun)
an orange of low brightness and saturation

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

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