Evan, I think it's a throw-back to the early pre-war and post war days. The overall 'Natural metal' first had white added to the top decking, to keep the cabin cool apparently, then the wings were painted grey to help prevent them looking too scruffy to the paying passengers, as the groundcrew would be all over them for re-fuelling etc, plus the oil, exhust and fuel stains etc. It seems to have developed from there, with the colours becoming 'designer' items in some cases. Plus, of course, the paint is also a protective coating, against corosion, important given the average service life of most airliners.
Starnge, we'll happily climb aboard and fly hundreds, or thousands of miles , on a thirty or forty year old airliner, but would think twice of driving a car of the same age!