WHITE COLOR?

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.

After a long time of building haze gray ships, i´m curently building Revells and Imex civil ships. And it is a long time since i was painting white suberstructures on ship models. The white that i use at that time was in the Humbrol range of colors. And after a couple of years it did not look good. Would not do the same mistake again, with som help from you in the forum. 

I seem to recall Humbrol white having that problem over twenty years ago. Paints have changed a lot since then.

As I said, it is usually the binder in a gloss white paint that yellows, or the clear coat over a flat white. My only suggestion is to paint flat white, and overcoat with Future, which is very resistant to yellowing.

UV (ultraviolet) is the big culprit. I managed to make Future yellow once by subjecting it to sunlight for three years.