This happens to my BO's pink-nosed Walker every summer. I'm pretty sure what is happening to your horses is just sunburn. The sunscreen may not be working because it is wearing off or isn't water proof (because they stick their noses into the water to drink and sometimes play around).
You could keep them in a stall during the day and turn them out at night, or (I prefer this option) get fly masks with a dropped nose, so it covers the pink. If you don't wan to do that right away, I'd go for some water proof, sweat proof, long lasting sunscreen and see if it helps. Aloe on the nose will help it heal.