How to whiten plastic plates ? They were white , now they're yellow

Something like Corell plates? Soak them in a sink with warm water and a cup of bleach. I put a toothpick between the plate or saucer edges to do several pieces at the same time. Best wishes ☺️

Try some bleach in some water. It might take a little soaking. Do one first and go from there as it will be a learning experience. DO NOT PUT BLEACH TO SOAK IN A STAINLESS SINK. USE A PLASTIC DISHPAN! Bleach left to soak stainless steel will pit it!

If they are plastic, could you just replace them more easily and possibly just as cheap? Try the second hand stores. They usually have many many plates for sale.

Put them in the sun for a day

Outside not in a sunny window. They need the UV rays. This will also bleach plastic storage containers stained with tomato products

Try bleaching them in a sink --- otherwise, they may be permanently discolored.