painlessness vs mildness

painlessness and mildness both are nouns.

Word	Noun	Adjective	Verb	Adverb
painlessness	Yes	No	No	No
mildness	Yes	No	No	No

As nouns, mildness is a hypernym of painlessness; that is, mildness is a word with a broader meaning than painlessness:

Other hypernyms of painlessness include gentleness, softness.

painlessness (noun)	mildness (noun)
The quality of being free from pain, without pain or trouble.	mercifulness as a consequence of being lenient or tolerant
Ease; lack of difficulty.	acting in a manner that is gentle and mild and even-tempered
	good weather with comfortable temperatures