In Italy, the basil is typically put on the pizza before baking, but the pizzas bake so fast that the basil still retains most of its nice green color. I personally put the basil on my pizzas after baking, either in whole leaves or chiffonade, and rely on the heat of the pizza to wilt the basil. But it stays nicely green. Sometimes, I do both--put some basil on the pizza before baking and some after. If you dip the basil in some of the pizza sauce before baking, that will also help slow down the loss of color. Otherwise, the basil can turn black on the pizza. I like the sharp and pungent flavor of really fresh basil so putting the basil on last satisfies that objective.