amber vs brownish-yellow

amber is a noun but brownish-yellow is not a noun.

Word	Noun	Adjective	Verb	Adverb
amber	Yes	Yes	No	No
brownish-yellow	No	Yes	No	No

As adjectives, amber and brownish-yellow are synonyms defined as:

Other synonyms of amber include yellow-brown.

amber (noun)	brownish-yellow (noun)
a deep yellow color
a hard yellowish to brownish translucent fossil resin; used for jewelry

amber (adjective)	brownish-yellow (adjective)
of a medium to dark brownish yellow color	of a medium to dark brownish yellow color