So your pink skinned horse spent a little too much time sunbathing? Some horses are more sensitive to the sun than others and can sunburn easily. The treatment for a sunburned horse is pretty similar to that of the sunburn treatment for humans: while the horse's sunburn is healing frequent applications of pure aloe can soothe the sunburned horse's skin immensely.
If the burn is around an eye it's best to not apply the sunburn treatment and allow nature to run it's course rather than risk introducing infection through a non sterile treatment application.
After your horse's sunburn has healed you'll want to consider ways to prevent sunburn in the future. First make sure your horse has access to shade, of course. Several products are on the market to prevent sunburn in horses. Flymasks and fly sheets provide UV protection to prevent sunburning, and zinc or other gentle forms of sunscreen applied to pink skin around the muzzle and face will prevent future sunburn.