If you've ever put a remote temperature sensor in unprotected sun, but away from heat-trapping areas like porches and decks, you can watch the temperature fluctuate by 30°F or more as the sun goes behind and out from clouds.

This is why the cooling sleeves work even when they're waterlogged ... they're thin enough that they don't put a barrier between the evaporative cooling and your skin; the fabric allows sweat to evaporate at at least the same rate it does from bare skin ... and no matter how pale you are or how sweat-yellowed the sleeves, they're still a lighter color.