A 2010 cross-cultural study - with participants from China, England, Germany, and the US - found that women are most attracted to men wearing red.
In one experiment from the study, 55 female undergrads looked at a color photo of a man in either a red or green shirt, and then rated the man's attractiveness.
Sure enough, the man was rated significantly more attractive when he was wearing a red shirt. The results were similar when researchers compared the red shirt to other color shirts as well.
Interestingly, participants generally weren't aware that the man's clothing color was influencing their perceptions of his attractiveness.