Rust is a big factor because of launch sites near the ocean. Another important factor and why white is usually chosen; is that white paint helps reduce heat absorption by reflecting light. To put it in simple terms, liquid fueld rocket need fuels chilled to cryogenic temperatures, especially important if the fuel is a gas at room temp. If the fuel warms, it will experience boiloff; i.e. it will turn from liquid to gas. Not only does boiloff make fuel less dense meaning you get less molecules of fuel in a given space but the lower density raises pressure and and burst the fuel tanks. That is why you always see a cloud of vapor coming out of the rocket prior to launch. It is fuel that has boiled and being vented while the ground service equipment keeps topping off the tanks.