Bleach turns coloured things white or clear, so it’s the perfect solution for clear silica gel phone cases.
Put on an apron, a disposable face mask and a pair of plastic gloves to protect yourself from irritating and corrosive bleach.
Dilute the bleach in proper proportions according to the manufacturer’s instructions and make sure the room you’re in is well-ventilated.
Soak the phone case in the bleach-water solution and brush it with an old, clean toothbrush.
Again, this won’t get rid of yellow stains because they’re often down to oxidation.