I actually asked a similar question over on Bimmerfest's detail department. According to those who replied, the yellowing doesn't come from paint fading. It's dirty and oxidation that has affected the clear coat. If a car is properly maintained in a reasonable amount of time with the proper protection (ie: sealants and wax), then it should stay white for years to come.

My mom who's a die hard "flat black" only type, continually tried to talk me out of getting a white car. Her main argument is that it turns yellow in time. I had to find out the truth and have a proper rebuttal. White cars are so easy to take care of! With only silver beating it out.