No one has gathered the data because anyone with half a brain understands that white clothes are more comfortable in the heat than black. Spending time and money on research of this nature would be a complete waste.
At some point, fashion desires must give way to intelligence. You didn't see the nomads running around the Sahara wearing black robes in daylight (even though they might have looked really sweet).
Actually a lot of them do wear black. The clothing absorbs heat from the sun more then white, but the wind would displace that heat before it reached the skin. As well the loose fitting nature of the clothing allows wind to reach the skin and keeps an air flow between the body and the skin, therefore reducing the body's temperature.
Loose fitting black clothing is better to wear in hot temperatures, if there is a consistent wind speed above 7 MPH.
It absorbs the heat better then White clothing if it's loose fitting as the wind will displace the heat before it reaches the skin. Were as white loose clothing is worse at displacing and preventing heat from getting to the skin, while it also keeps heat in as well.
In conditions with out much wind (under 7 MPH) White is obviously a better color to wear as the black cloth would absorb the heat and displace it into the skin. (For Tight fitting white clothing, with no wind they are better at displacing heat from the sun. However, heat that radiates from the body is keep in by white clothing more then black as the white reflects the heat back to the skin.)
That's based off of scientific research on mammals, birds and clothing. Where as the information based on nomads wearing black is from a few scientific journals.
The study on birds was done by (Walsberg, Campbell, & King, 1978. J. Comp. Physiol.)
As for the debate about NFL jersey's, I'd say it would make a difference and the athlete's would notice it. The effect would be reduced by somewhat by the fact that the Athletes have access to IV's and consistent refreshment, but not completely. The body still can only take so much heat in with out proper displacement before it starts to cause harm.
Helmet color would have the largest impact as most of the heat that escapes from the body does so from the head, a white or similarly reflective color would be ideal for helmets. Reflecting a large amount of heat from the body, keeping body temperatures lower. A black helmet would increase the body temperature by at least a few degrees (if not more) in rigorous activities were heat would be a factor.
Edited by carpanfan96, 24 February 2012 - 02:49 AM.