Lighten up. Your preschooler is mimicking the person she most looks up to -- you. It's no different than her "cooking dinner for her babies" (a.k.a. making imaginary pies for her dolls).
I understand, though. I groaned when Lila's godmother gave her a box of play makeup for her fourth birthday. Lila's eyes shone, but I wasn't ready to see my baby slather blue eye shadow and red lipstick all over her face. So when she wasn't looking, I took out the offending items and replaced them with clear lip glosses and body lotions. These satisfied her desire to put on makeup but were more age-appropriate.
Chances are, if you give her your lip balm and an old fluffy brush, she'll forget about the makeup after a while and move on to a new toy. But if she doesn't, or can't walk by a tween makeup haven like Club Libby Lu or Claire's without begging to go in, hip her to your rules: Yes, it's fun to pretend, but the real stuff is for grown-ups.