From New Scientist:
The powerful influence of colour on sporting success was first discovered a few years ago, when evolutionary anthropologists Russell Hill and Robert Barton of Durham University, UK, were looking for some way to test the idea that colours influence human behaviour. The 2004 Athens Olympics were coming up, and it dawned on them that in some Olympic combat sports - boxing, taekwondo, Graeco-Roman wrestling and freestyle wrestling - competitors are randomly assigned a red or blue kit. "We realised that this was a ready-made experiment to study the effects of colour on match outcome," Barton says.
When they analysed the results they found that shirt colour appeared to influence the result, with nearly 55 per cent of bouts being won by the competitor in red. In closely fought bouts it was 62 per cent (Nature, vol 435, p 293). "It should have been roughly 50 per cent red, 50 per cent blue, and this was a statistically significant deviation," Barton says. "Skill and strength may be the main factors - if you're rubbish, a red shirt won't stop you from losing, but when fights were relatively symmetrical, colour tipped the balance."
Barton says that the differences may be accounted for, to some extent, by a referee's unconscious preference for red - which he argues is an inherited preference - as seen in the taekwondo experiment. He also believes colour affects the combatants' mood and behaviour. "There is now good experimental evidence that red stimuli are perceived as dominant and that they cause negative effects on performance in those viewing them," Barton says. "It is plausible that wearing red also makes individuals feel more confident, although this hasn't yet been tested."