Filler reacting with the Sun.

Hey I was wondering if anybody has any prob with filler changing color after painting, I have had one truck painted and I painted another truck my self white and were the filler is after a while it seems like it has a yellow tint to it. some people tried to say it was the blue preal in it. Im getting to repaint the truck that I had painted and I dont want it to do it again any sugestions.Thanks.