sombreness vs earnestness

sombreness and earnestness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sombreness	Yes	No	No	No
earnestness	Yes	No	No	No

As nouns, earnestness is a hypernym of sombreness; that is, earnestness is a word with a broader meaning than sombreness:

Other hypernyms of sombreness include serious-mindedness, seriousness, sincerity.

sombreness (noun)	earnestness (noun)
a manner that is serious and solemn	the trait of being serious
a feeling of melancholy apprehension	an earnest and sincere feeling
a state of partial or total darkness