There's this really cool series by James Clemens.
Wit'ch Fire is the first one...
Point is, magic there is different for each person, the protagonist gets it by exposing her hand to light and absorbing it. When she does that, her hand becomes blood red. As she uses the magic inside (by cutting her hand and draining her blood) her hand becomes paler. She can only renew it when her hand becomes pale, she needs to find a source of natural light to absorb from.
Differnt light gave the magic different properties: Sun= Fire, Moon=Ice, "Ghost light"= invisibility or when she smeared her eyes with her own "ghost light" blood, she'd see sources of magic.
I dunno, I thought it was an uber kool idea, so I threw it out there.