I prefer red, but any color is fine. Wearing lipstick turns me on.
I especially like seeing men in lipstick. Robert Smith (The Cure), Eddie Izzard (comedian), Marilyn Manson (...sometimes), Tim Curry in The Rocky Horror Picture Show... all great examples of great lipstick wearers.
On girls, my reaction varies. I think Brody Dalle was extremely hot in lipstick, before she became a mom and changed her looks and all. Some girls I like better without lipstick, though.
I've always wanted to kiss a man wearing lipstick while wearing it myself. For some reason, I think that would just be amazing. I've never been with a guy that would do it, though.