dullness vs dimness

dullness and dimness both are nouns.

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

As nouns, dimness is a hyponym of dullness; that is, dimness is a word with a more specific, narrower meaning than dullness:

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

Other hyponyms of dullness include subduedness, flatness, lusterlessness, lustrelessness, mat, matt, matte.

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