sombreness vs seriousness

sombreness and seriousness both are nouns.

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

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

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

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