plainness vs simplicity

plainness and simplicity both are nouns.

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

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

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

plainness (noun)	simplicity (noun)
the appearance of being plain and unpretentious	freedom from difficulty or hardship or effort
an appearance that is not attractive or beautiful	lack of ornamentation
clarity as a consequence of being perspicuous	the quality of being simple or uncompounded
the state of being unmixed with other material	absence of affectation or pretense
	a lack of penetration or subtlety