In USAAF units, camouflage paint schemes were controlled by the theater commander. When there was a credible threat of enemy attack, they kept paint on the aircraft so while they were on the ground, the would blend into the terrain. So, you'll note that through 1942-1944 in ETO/MTO, most USAAF aircraft maintained camo paint schemes. IN the Pacific, it lasted even longer, depending on theater. Once the Allies had established air supremacy in their respective theaters, they began to leave the aircraft unpainted, but it wasn't for performance reasons, otherwise they would have polished the aluminum. For the Navy/Marine Corps, corrosion control was the primary reason the aircraft were painted. Even today, corrosion prevention is a huge part of the aircraft maintenance effort, as aircraft that are exposed to the flight deck of a carrier or amphibious assault ship are constantly exposed to salt water.