sunless vs overcast

sunless is not a noun while overcast is a noun.

Word	Noun	Adjective	Verb	Adverb
sunless	No	Yes	No	No
overcast	Yes	Yes	Yes	No

As adjectives, sunless and overcast are synonyms defined as:

Other synonyms of sunless include cloud-covered, clouded.

sunless (noun)	overcast (noun)
	a cast that falls beyond the intended spot
	a long whipstitch or overhand stitch overlying an edge to prevent raveling
	the state of the sky when it is covered by clouds
	gloomy semidarkness caused by cloud cover

sunless (adjective)	overcast (adjective)
filled or abounding with clouds	filled or abounding with clouds

sunless (verb)	overcast (verb)
	sew with an overcast stitch from one section to the next
	sew over the edge of with long slanting wide stitches
	make overcast or cloudy