Exposure to sun is the main cause causing the tanning
of the skin and the area which is covered is left. The best method the prevent from this is to cover your body as much as possible when traveling in the sun. Clothing is the best sunscreen to prevent from the tanning. Otherwise use sunscreen cream on the exposed area specially on face and arms which will prevent sun burn and also prevent discoloration. As far as the discoloration is concern this will get back to normal after a few weeks itself when new skin will be formed, you just need to prevent yourself from sun exposure or use proper clothing and sunscreen when going out in the sun. Other wise you can use lemon, tomato paste and papaya at home to get ride of tanning fast, but one thing you have to always do is to prevent yourself from direct sun exposure.
Thanks and take care.