overcast vs sunless

overcast is a noun but sunless is not a noun.

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

As adjectives, overcast and sunless are synonyms defined as:

Other synonyms of overcast include cloud-covered, clouded.

overcast (noun)	sunless (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

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

overcast (verb)	sunless (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