light up has 8 distinct synonyms spread across 3 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.
start to burn with a bright flame
“The coal in the BBQ grill finally lit up”
make lighter or brighter
“This lamp lightens the room a bit”
ignite
“The sky lit up quickly above the raging volcano”
Synonyms, antonyms and the broader and narrower relations for light up come from Princeton University’s WordNet. Full sources and licences.