purity vs plainness

purity and plainness both are nouns.

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

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

Other hyponyms of purity include kosherness.

purity (noun)	plainness (noun)
a woman's virtue or chastity	the appearance of being plain and unpretentious
being undiluted or unmixed with extraneous material	an appearance that is not attractive or beautiful
the state of being unsullied by sin or moral wrong; lacking a knowledge of evil	clarity as a consequence of being perspicuous
	the state of being unmixed with other material