Men prefer women with symmetrical facial features. Men also tend to be attracted to women with hourglass waist-hip ratios and particular body-mass index. As one scientific study informs us, men also tend to find women especially attractive when the bar is just about to close. In fact, psychologists have identified many (often very subtle) cues that seem to generally trigger male heterosexual attraction. Usually these cues come in the form of physical characteristics with more-or-less direct relation to reproductive fitness, but sometimes, as a recent study in the journal of personality and social psychology reports, the story is a little more colorful:
Like many of our primate relatives, it seems that male humans show increased sexual desire for women when exposed to the color red. In particular the mere presence of red in the context of viewing pictures of moderately attractive women tends to increase men's ratings of a woman's overall attractiveness, own perceived sexual desire, as well as dating and spending intentions.
Actually, the finding would be less interesting if we were talking about an aphrodisiacal effect of a red flush (a possible sign of arousal), red lipstick (possible to be mistaken as a sign of arousal), or even red shoes. However, the study explicitly shows that it is the color itself that increases desire, and that it does so without being directly related to any feature of the female's appearance. The researchers are able to make this distinction because participants in the study were actually rating black-and-white pictures of females; some of which were presented over a red pas partout, others which had other colors for a background. Yet, the effect of the red stimulus was quite stunning (as the graphs below show).

To illustrate the unique standing of this result, imagine that men found women more attractive when viewing their picture next to a photograph of a hourglass. Although I don't think anyone has tried this, it seems unlikely that the resemblance of the hourglass shape to men's preferred waist-hip ratio would cause men to be more excited about the woman.
The study shows that men are, as with so many things, unaware of the color's effect on their own preferences. Moreover the effect does not occur for women rating men, nor for women rating other women.
A plausible hypothesis for the effect is that there is an overlap of the evolutionary meaning of red for the male mind as well as the learned cultural association. Hence the effect is viewed as
"emerging from both societal use of red and a biologically engrained predisposition to red. These two sources may contribute to the red-sex link in joint fashion."
And just in case you were wondering, "the red effect does not extend to general positive characteristics" nor did color affect kindness or intelligence ratings.