Light makes colors fade because ultraviolet radiation is capable of breaking down the molecules of pigments and dyes. The long molecules that make up these colors are often unstable, and the energy that ultraviolet radiation strikes these molecules with is capable of breaking the weaker molecular bonds in the structure. This degrades the colors over time, and as more molecules break down, colors begin to fade.
How quickly a color fades in sunlight depends on a number of factors. The most important factor is the chemical makeup of the dye or pigment itself. Certain colors are made up of relatively stable molecules, making it difficult for ultraviolet radiation to break the bonds and providing long-lasting defenses against fading. Others, especially those created to be environmentally friendly, may be more unstable and easier to break down when exposed to light. Over time, virtually all pigments fade in the presence of light.
This problem is why many important artifacts, such as paintings or documents, are kept in an environment with controlled lighting and ultraviolet protection. The Declaration of Independence and the U.S. Constitution are protected under glass panes designed to block as much ultraviolet radiation as possible. Even something as simple as a photo album often contains UV-blocking capabilities that protect personal memorabilia from the effects of sunlight.