colourless vs drab

colourless is not a noun while drab is a noun.

Word	Noun	Adjective	Verb	Adverb
colourless	No	Yes	No	No
drab	Yes	Yes	No	No

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

colourless (adjective)	drab (adjective)
weak in color; not colorful	of a light brownish green color
lacking in variety and interest	lacking brightness or color; dull
	causing dejection
	lacking in liveliness or charm or surprise