Oxidation of the surface of the paint. This was much more common before solid colours such as red were clearcoated to help prevent surface oxidation, but, as you know, it's quite common for the surface clearcoat lacquer to peel off and for the paint to fade. All colour coats now have to be water-based paint. The EU 'environmental' rules on this have created a huge business in aftermarket wax and ceramic coating of cars the environmental effects of which are not clear.