You could say "lighten up" meaning don't be so mean. Or "lighten the mood" as in bring the energy or positivity up.

You're right. Lighten and brighten are quite similar, especially when it used to describe any situation with light. Lighten also has other meanings, like to make less heavy, but you probably know them already. ;) However, many times lighten is more associated with color and to lighten something means to adjust the shade of color towards the white end of a spectrum. Brighten doesn't really address color as much and is more descriptive of the actual increase of light. It often means to make something more brilliant. LIGHTEN: 1. All the time in the sun has lightened her hair. 2. The BB cream has lightened her skin. BRIGHTEN: 1. That shirt really brightens up your face! 2. The sunlight brightened the attic.

One can lighten a color by painting over the color with one of a less dark paint. You can brighten a room by turning on the lights in that room. A person can brighten their expression by smiling (idiomatic). You can brighten a metal object by polishing it.