v.1. To become brighter or more intense in color: The sky brightened up quickly as the clouds dispersed.
2. To make something brighter or more intense in color: Some red paint will brighten up this dreary room. Put some yellow into the dye to brighten it up.
3. To become happier; improve one's mood or outlook: He brightened up after you began talking with him.
4. To make someone happier or more cheerful: The sunny weather brightened up the tired travelers. A cool glass of lemonade would brighten me up right now.