dullness vs flatness

dullness and flatness both are nouns.

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

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

dullness: a lack of visual brightness
flatness: the property of having little or no contrast; lacking highlights or gloss

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

dullness (noun)	flatness (noun)
lack of sensibility	the property of having two dimensions
without sharpness or clearness of edge or point	inactivity; showing an unusual lack of energy
the quality of lacking interestingness	the property of having little or no contrast; lacking highlights or gloss
a lack of visual brightness	a deficiency in flavor
the quality of being slow to understand	a want of animation or brilliance