sepia vs reddish brown

sepia and reddish brown both are nouns.

Word	Noun	Adjective	Verb	Adverb
sepia	Yes	No	No	No
reddish brown	Yes	No	No	No

As nouns, sepia and reddish brown are synonyms defined as:

Other synonyms of sepia include burnt sienna, mahogany, Venetian red.

sepia (noun)	reddish brown (noun)
a shade of brown with a tinge of red	a shade of brown with a tinge of red
rich brown pigment prepared from the ink of cuttlefishes