The sky 'lightened' [='brightened'] as the clouds began to move away but darkened again when the clouds returned.
a hair/skin 'lightener' [=a chemical or dye used to make a person's hair or skin less dark]
Her somber mood gradually 'lightened' [='brightened'] as the weather improved.
(..) I'll give you half. Mom! Get some food. No sense of humor. And that's another thing: lighten up a bit. This severe routine is getting old. You're acting like such a geek. Smile once in a while. Smile? Yeah. You know, smile. (..)1991 Terminator 2: Judgment Day