flatness vs dullness

flatness and dullness both are nouns.

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

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

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

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