severity vs plainness

severity and plainness both are nouns.

Word	Noun	Adjective	Verb	Adverb
severity	Yes	No	No	No
plainness	Yes	No	No	No

As nouns, plainness is a hypernym of severity; that is, plainness is a word with a broader meaning than severity:

severity (noun)	plainness (noun)
excessive sternness	the appearance of being plain and unpretentious
used of the degree of something undesirable e.g. pain or weather	an appearance that is not attractive or beautiful
something hard to endure	clarity as a consequence of being perspicuous
extreme plainness	the state of being unmixed with other material