Yellow stains can be removed from white plastic by using bleach, sand paper or commercial cleaner. For extremely difficult stains, a combination of the three methods can be used.
The most passive method for cleaning white plastic that is stained yellow is to use a bleach soak. The items can be placed in a solution that is one part bleach and one part water. It is generally safe to soak the items until they turn their original white color. They can then be rinsed and air dried.
Sandpaper can be used for stains that are difficult to remove with bleach. The goal of using sandpaper is to lightly buff off the top layer of the plastic where the stains exist. The fine grit of the sandpaper should be rubbed on the plastic, but it should not be used too harshly because the plastic can be easily worn down. The residue from the sandpaper can be washed off with water.
A commercial cleaner can be used to remove yellow stains from the plastic. It is important to make sure that the commercial cleaner is safe for use on plastic before it is used to remove the stain. It should always be rinsed off after being used.