Any wrap is going to create a layer of insulation that makes it more difficult to shed internally generated heat so even a highly reflective color may not provide much net gain. My Canon "great white" primes get plenty hot when shooting baseball in the sun with no significant internal heat generation so the reflective white helps but it doesn't result in a cool touch.
If you have staked out a clear space without other photographers around, then a stationary umbrella is the best friend to you and your camera. If you are in the mosh pit of photographers, then you and your camera are going to have to sweat it out.
A couple of times I have used Canon's camera "rain suits" to protect my 1 series cameras and long lenses in absolute pouring rain. Heat isn't an issue with them but any sort of covering or suit added to the camera makes it less fun to handle and I typically shoot with two and sometimes three bodies so I really hate using any sort of camera cover because a single added second in switching between bodies because of an awkward cover can easily result in missed killer shots.
With the amount of heat generated in MILC architecture, Canon and others need to consider a couple of models with active cooling systems for those who shoot under high ambient/direct sun conditions. These have been common to video cameras for a long time, the downsides are increased power consumption and reduced rain/moisture resistance.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
