According to my father, who was a pilot for Eastern Airlines, Eastern went from painting their planes white to polished aluminum to save the cost of repainting, save weight and fuel, and also increase the revenue payload of the plane. The only plane they couldn't do that with where the A-300's. They didn't have the alclad aluminum skin like the Boeing planes, thus had to be painted for corrosion protections (considering they spend their life, for the most part, flying the east and Caribbean. A friend who flies for Evergreen (sits sideways on 747's) said the most of them are now parked in the desert and are unairworthy due to excessive corrosion from the salt air environment they lived in.

Hogflyer

Our FedEx planes have bare wings too for the standard reasons. Our Airbus wings are painted to Airbus specs and not in FedEx colors. As for the corrosion, I con't have to work on that thankfully as I work in a back shop. One of the reasons we changed our color schemes in our last paint scheme makeover was to use more white than purple/orange. Less pigment in the paint which again saves weight and the white lasts better in the sun.