It's an ongoing question for many men and women as to why red lipstick is sexy. Now there are occasions where red simply doesn't work for a woman, but usually there's at least one shade that will add to a woman's sex appeal. Now, red lipstick does draw attention to a woman's lips, and it can make the lips appear fuller and plumper. Given that studies have shown men pay more attention to a woman's mouth than any other part of her face, that can go a long way toward making quite an impression. However, even with all the psychological studies and research into color there are still men (and women as well) who find this look incredibly sexy, they don't know why.
It's because their bodies know what their mind may not consciously be aware of.
What Your Lower Brain Remembers
That Your Upper Brain Has Forgotten
It's fairly simple actually; red lips mean the owner is aroused.
When the blood in a woman's lips moves closer to the surface of the skin it's an indicator that she's turned on (and in some cases that she's just had an orgasm). Men get turned on by this fact because, while their conscious minds might be thinking about the girl's skin tone and how that lipstick is really bright, deep down their biological imperative recognizes the sign that the girl is sexually aroused, and thus you have a better chance of getting with her. It's sort of like pheromones, which are the chemicals released through your sweat when you get aroused to help entice the opposite sex. You smell them, but you aren't aware that it's the pheromones the member of the opposite sex is giving off. All you know is she, or he, smells good to you, and you want to experience more.
The idea of mimicking natural arousal is in fact the goal of most makeup, whether it's modern or ancient; red lips are only one example. Blush is meant to represent the natural flush of arousal is your cheeks that can make you look more vibrant, and women in Italy used to put liquid belladonna in their eyes to make the pupils dilate, because that's another feature which was found attractive (in case you missed the theme, your pupils dilate when your aroused). So, whether you find red lipstick to be sexy or trashy, there's a reason it works so well and why so many women will put it on before they go out.