dimness vs dullness

dimness and dullness both are nouns.

Word	Noun	Adjective	Verb	Adverb
dimness	Yes	No	No	No
dullness	Yes	No	No	No

As nouns, dullness is a hypernym of dimness; that is, dullness is a word with a broader meaning than dimness:

dimness: the property of lights or sounds that lack brilliance or are reduced in intensity
dullness: a lack of visual brightness

dimness (noun)	dullness (noun)
the property of lights or sounds that lack brilliance or are reduced in intensity	lack of sensibility
the quality of being dim or lacking contrast	without sharpness or clearness of edge or point
the state of being poorly illuminated	the quality of lacking interestingness
	a lack of visual brightness
	the quality of being slow to understand