There is a lot of science, experimentation and trail and error involved in this, mostly done by plastics-, pigment- and paint manufacturers. Some remarks:
Under intense light (UV), color pigments break down, and whites or clears go yellow. A pink jem will end up orange if it comes out of its case too much. A white UV goes yellow, some people like this effect ('vintage white') others, like you, don't
Some plastics/coatings/pigments are UV resistant, but they can also UV transparent! So an acrylic won't go yellow, but the pigments inside can still break down.
Carbon particles (soot) are a good way to absorb UV, so dark colors are better (plus the yellowing will show less, duh...)
A common pigment for whitening is Titanium Dioxide, it's also very UV resistant:
http://en.wikipedia.org/wiki/Titanium_dioxide
The combination of an acrylic paint (PMMA UV transparant) and TiO2 will for instance give you a stable white.
Once the pigments have broken down, or the clears have yellowed you cannot go back. You can only refinish.
