How to clean dirty white tiles?
Check out this previous question. There is a solution called "retr0brite" in which you make a chemical compound that you apply to the yellowed bricks, leave them exposed to light for a while, and the bricks will come out clean white. However, I have recently read (but cannot find the link right now) that this retr0brite method might not work forever, and that with reapplication the duration of the whitening reduces every time.