reddish brown vs burnt sienna

reddish brown and burnt sienna both are nouns.

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

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

Other synonyms of reddish brown include mahogany, sepia, Venetian red.

reddish brown (noun)	burnt sienna (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