Color Fading of Plastics

Shade them from sunlight?

Specifically from UV

Use light fast pigments to color plastics

UV light, even from fluorescent lights, will fade or yellow many plastics. Work with a color compounder of plastics to get fade resistant plastics. Some colors resist fading more than others. With RoHS and other recent regulations, the choices of possible pigments is much more reduced than in the past, making it more difficult to match colors and to make them fade resistant.

The more expensive plastics are using a UV absorber to minimize fading of the color.