Good coat spray with sunscreen or a flysheet. I use Absorbine's Santa Fe spray on my appy who has some pink skin on his rump and it seems to work pretty well. If you get a fly sheet, look for one that blocks 70% or upwards of UVB, preferably with a neck cover.
Definitely if they are out in the sun, try keeping a fly sheet on them. I usually turn my horses out at night during the summer to protect from bleaching and bugs. I have also heard good and bad things about supplement, just be sure that what you use will not test positive in a drug test