There are several things you can use to remove a yellow stain or discolouration from white plastic, but what you use depends on the object that has the stain. Some things, such as computers, need a gentle whitener because harsh chemicals may cause damage to delicate parts. You can try starting with household products or purchase a product made specially for removing stains from plastic. Simple items you probably already have in your kitchen can also do the trick.
Bleach is effective on any kind of white plastic that can be soaked directly in a water and bleach mixture. Depending on manufacturer recommendations, you may sometimes be able to use undiluted bleach to remove stains. Because bleach is such a strong chemical, you should first try diluting it by mixing a small amount of it with a large amount of water.
Sometimes the bleaching will completely take care of the stain and sometimes a little bit of scrubbing is needed. Use a plastic wire brush or a sponge and be careful with steel wool, as it can leave micro-abrasions depending on the plastic. Because of the fumes, always keep the area well-ventilated when you are working with bleach, or wear a face mask.
Make a paste of baking soda and water, apply it to the surface of the plastic and leave it on for a few hours. Baking soda is a gentle whitening agent that gets rid of most food stains on hard surfaces, and it is safe to use on delicate.materials.
White vinegar is also useful for removing yellow stains from white plastic and is natural and gentle, but it gives off an odour. White vinegar can be diluted to lessen the smell, but it isn't necessary and will lessen the effectiveness. If soaking in these agents does not remove the stain, try using a toothbrush or other nylon-bristled brush to scrub the area.