brown vs dark-brown

brown is a noun but dark-brown is not a noun.

Word	Noun	Adjective	Verb	Adverb
brown	Yes	Yes	Yes	No
dark-brown	No	Yes	No	No

As adjectives, brown and dark-brown are synonyms defined as:

Other synonyms of brown include brownish, chocolate-brown.

brown (noun)	dark-brown (noun)
an orange of low brightness and saturation

brown (adjective)	dark-brown (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)	dark-brown (verb)
fry in a pan until it changes color
make brown in color