I'm not sure, but I picked black. It's a neutral color and it has a great contrast with many bright colors. "Mysterious and dark" are what I consider sexy, I guess. When I look at red, I think "lust." I do understand that most people view red as sexy though, since red is associated with passion and spiciness, Valentine's Day, and the heart.
edit: It also just popped into my mind that red is hot.