Question for painters

[Hook'em Horns]

I've been painting for several years now and still after years of experience there is something that I still cant figure out. After a year or so from the time the team is finished I start to see a yellowish tint on teams that have a lot of white in their uniforms.

Let me say "Yes" before you ask me the question...Do I paint on the white? I do because the figure gets a cleaner look once complete. However, it never fails after several months I start to see a yellowish tint on the white portion of the uniform.

Any other painters have this problem and if so what have you done to prevent this from happening?

Thank you for your help.