You can use a little bit of lemon juice or a little bit of a clear soft drink, such as Sprite or 7-UP to preserve the color of your fruit. The citric acid is what keeps the apple from turning brown. You can also buy a product called "fruit fresh" in the grocery store, which is a powder you can sprinkle on fresh fruit, but why spend extra money when you don't need to?
BTW, good for you for brown-bagging it and eating healthfully as well!