dark-brown vs brown

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

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

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

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

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

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

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