olive-drab vs drab

olive-drab is not a noun while drab is a noun.

Word	Noun	Adjective	Verb	Adverb
olive-drab	No	Yes	No	No
drab	Yes	Yes	No	No

As adjectives, olive-drab and drab are synonyms defined as:

olive-drab (noun)	drab (noun)
	a dull greyish to yellowish or light olive brown

olive-drab (adjective)	drab (adjective)
of a light brownish green color	of a light brownish green color
	lacking brightness or color; dull
	causing dejection
	lacking in liveliness or charm or surprise