burnt sienna vs reddish brown

burnt sienna and reddish brown both are nouns.

Word	Noun	Adjective	Verb	Adverb
burnt sienna	Yes	No	No	No
reddish brown	Yes	No	No	No

As nouns, burnt sienna and reddish brown are synonyms defined as:

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

burnt sienna (noun)	reddish brown (noun)
a shade of brown with a tinge of red	a shade of brown with a tinge of red
a reddish-brown pigment produced by roasting sienna