glum vs sullen

glum and sullen both are adjectives.

Word	Noun	Adjective	Verb	Adverb
glum	No	Yes	No	No
sullen	No	Yes	No	No

As adjectives, glum and sullen are synonyms defined as:

Other synonyms of glum include dark, dour, glowering, moody, morose, saturnine, sour.

glum (adjective)	sullen (adjective)
moody and melancholic	darkened by clouds
showing a brooding ill humor	showing a brooding ill humor