Was a corrosion issue, at least eventually. After the war, in the early 50s, they tried making bare metal standard - -but experiment only lasted a brief time before it became obvious that corrosion was a major issue, and more than outweighed any benefits.
During the war, not sure. The USN was far more strict about markings and colors than the USAAF, which is why the nose art is so muted on naval plaens. Might be that they were just so conservative that they never considered the change. In the USAAF, the change was made for weight and speed reasons.