plainness vs starkness

plainness and starkness both are nouns.

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

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

Other hyponyms of plainness include chasteness, restraint, simpleness, simplicity, austereness, severeness, severity, bareness.

plainness (noun)	starkness (noun)
the appearance of being plain and unpretentious	an extreme lack of furnishings or ornamentation
an appearance that is not attractive or beautiful	the quality of being complete or utter or extreme
clarity as a consequence of being perspicuous
the state of being unmixed with other material