Proven wrong!
Forfeit: Light coloured clothes
Answer: It doesn’t matter, just wear something light and loose if you want to stay cool.
It is strange that Bedouins, who live in hot deserts, wear black robes, but a study done by Tel Aviv University and Harvard has shown that the amount of heat gained by a person in the desert is the same whether he wears a black or a white robe. The additional heat absorbed by the black robe was lost before it reached the skin.
A man stood in the desert for 30 minutes, facing the sun four times, once in a black robe, once in a white one, once in an Army uniform and once half-naked. The results were surprising, they found that the man was equally cool irrespective of the colour of his robe. It worked like a chimney, because the air is hotter underneath the black robe, it rises more quickly up and out of the loose robe and, by convection, is replaced by the cooler air beneath it.
link