It is all about the chemical makeup of an object. The technical term for color fading is photodegradation. There are light absorbing color bodies called chromophores that are present in dyes. The colors we see are based upon these chemical bonds and the amount of light that is absorbed in a particular wavelength.
Bright sun. Palo Alto Battlefield National Historical Park, 2017. National Park Service, NPGallery.
Ultraviolet rays are one of the causes of fading because they can break down chemical bonds and fade the color in an object. Other major contributors to fading include visible light and solar heat.
Some objects may be more prone to this bleaching effect, such as dyed textiles and watercolors. Other objects may reflect the light more, which makes them less prone to fade.
