plainness vs pureness

plainness and pureness both are nouns.

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

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

Other hypernyms of plainness include purity.

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