lighten vs mitigate

lighten and mitigate both are verbs.

Word	Noun	Adjective	Verb	Adverb
lighten	No	No	Yes	No
mitigate	No	No	Yes	No

As verbs, mitigate is a hypernym of lighten; that is, mitigate is a word with a broader meaning than lighten:

lighten (verb)	mitigate (verb)
make more cheerful through the use of color	to temper
become lighter	lessen or to try to lessen the seriousness or extent of
alleviate or remove (pressure or stress) or make less oppressive
become more cheerful
reduce the weight on; make lighter
make more cheerful