Originally Posted by mdj1

I believe that's actually incorrect.

Most planes are painted white to decrease heat build up. The skin of a plane already gets hot through friction during flight and dark colours increase this heat. The hotter the plane the more fuel it burns. So yes it is about cost, but for different reasons.

In fact, when Concorde was painted in the pepsi colours they were advised not to take it above mach 2 for more than 20 minutes as they were worried what the additional heat would do to the fuselage.