Leading the list of decay is oxidation. In the same way our skin becomes burned by overexposure to the sun, a painted surface will become damaged if it's not protected from the sun's UV light. These destructive light rays attack the paint surface with ultraviolet radiation and degenerate the paint's molecular components. This degeneration allows oxygen to combine with the paint's polymers and oxidation occurs. Generally, the effects will be seen in the form of fading or clearcoat breakdown.
Metallic paints seem to suffer much worse than others, especially silver.
Red paint also has a high frequency of fading due to having more chemical components required to develop its color than natural colors. Another paint destroyer is pollution. Large metropolitan areas are sometimes plagued with air pollution so bad that smog alerts are issued to warn the public. Pollution eats away at everything and you can't even see it happening. In coastal regions, salt air is especially detrimental because it allows the microscopic metals found in the air to oxidize or rust more rapidly upon contact with your vehicle's finish.