hardness vs dullness

hardness and dullness both are nouns.

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

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

hardness (noun)	dullness (noun)
a quality of water that contains dissolved mineral salts that prevent soap from lathering	lack of sensibility
the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale	without sharpness or clearness of edge or point
the quality of being difficult to do	the quality of lacking interestingness
devoid of passion or feeling; hardheartedness	a lack of visual brightness
excessive sternness	the quality of being slow to understand