At the temperature you're baking at the specific brand of cheese that you are using in browning excessively, so when you bake the pizza right on the stone you are getting the bottom done faster than you would if using a screen. This means that the top and bottom of the pizza are being baked more closely together. With the screen, the baking time will be longer, but the cheese can't be subjected to a longer bake or it will burn. Increasing the baking temperature will only result in the cheese burning faster, so the solution is, when baking on a screen you must actually reduce the baking temperature to something in the 500F/260C range and bake the pizza for a longer time. If you find that the cheese still gets too dark, you might need to change to a different cheese manufacturer.
Tom Lehmann/The Dough Doctor