It’s a dry heat.
Seriously, this time of year the heat is due to Katabathic wind, or what we would call Santa Ana wind. Essentially, this is a drainage wind that carries high density air from a higher elevation down a slope under the force of gravity. In the descending process the air is compressed and heats up. Such winds are sometimes also called fall winds. So the heat is less due to direct sunlight and more do to the effect of Santa Ana winds. As a result, the black color of the helmets would have minimal impact on the heat felt by the players.