You should be able to get un-tan if you spend minimal time in the sun, or when you do spend time in the sun, use an umbrella. It'll go away after a few days if you do that.
Ohh, I have this problem so badly!! I don't even enter the sun, I only play at night, and only enter the sun very occasionally outside of the island, and despite all this I am nearly black!! XD It's becoming very frustrating, maybe it's moonburn? LOL I don't think I should have to carry around an umbrella at night XD