plainness vs lucidness

plainness and lucidness both are nouns.

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

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

plainness: clarity as a consequence of being perspicuous
lucidness: free from obscurity and easy to understand; the comprehensibility of clear expression

Other hypernyms of plainness include clarity, clearness, limpidity, lucidity, pellucidity.

plainness (noun)	lucidness (noun)
the appearance of being plain and unpretentious	free from obscurity and easy to understand; the comprehensibility of clear expression
an appearance that is not attractive or beautiful
clarity as a consequence of being perspicuous
the state of being unmixed with other material