simplicity vs plainness

simplicity and plainness both are nouns.

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

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

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