dullness vs insipidness

dullness and insipidness both are nouns.

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

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

dullness (noun)	insipidness (noun)
lack of sensibility	lacking any distinctive or interesting taste property
without sharpness or clearness of edge or point	extreme dullness; lacking spirit or interest
the quality of lacking interestingness
a lack of visual brightness
the quality of being slow to understand