you cant, old things like plastic turn colour with dirt and age. sometimes you must buy something new, you cant cover it up or it will just go yellow again. and nothing will cover it up anyway. buy a cheap new washing machine.

Use soap powder made for white clothes, rub against the stain and then rinse - soap powder has whitners in it - worked on the old washing machine that was left in my old house - my mother in law passed that one on to me!