mellowness has 2 distinct synonyms spread across 1 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a feeling of good humor and sympathy through maturity or intoxication or a relaxed state
a taste (especially of fruit) that is ripe and of full flavor
the property of a sensation that is rich and pleasing
“the music had a fullness that echoed through the hall”
Synonyms, antonyms and the broader and narrower relations for mellowness come from Princeton University’s WordNet. Full sources and licences.
a soft shade of a color
“a mellowness of light and shade not attainable in marble”
geniality, as through the effects of alcohol or marijuana