copper color vs reddish brown

copper color and reddish brown both are nouns.

Word	Noun	Adjective	Verb	Adverb
copper color	Yes	No	No	No
reddish brown	Yes	No	No	No

As nouns, reddish brown is a hypernym of copper color; that is, reddish brown is a word with a broader meaning than copper color:

Other hypernyms of copper color include burnt sienna, mahogany, sepia, Venetian red.

copper color (noun)	reddish brown (noun)
a reddish-brown color resembling the color of polished copper	a shade of brown with a tinge of red