dullness vs hardness

dullness and hardness both are nouns.

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

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

Other hypernyms of dullness include callosity, callousness, insensibility, unfeelingness.

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