What paint are you using, and are you overcoating it with anything?
Often, it is the clear coat on top that yellows, due to UV degradation or oxidation, or both. Some gloss whites also have the same problem.
I use Tamiya acrylics, and have never had the problem with the paint. Future will yellow if exposed to strong UV for long enough.
Artist's oil paint (often used on figures) will also yellow due to oxidation and UV degradation of the drying oil.