Why do mobile covers turn yellowish

I am a hair dresser, and after I would have someone get highlights in their hair, I would often have leftover bleach. Not wanting it to go to waste, I would clean my white tile floor and grout…it worked great. Then, I had some yellowing and some rust stains on my countertop; I used it there too. Recently, when there was the rusty/orange-colored stains on my hot tub, I used it on there; hot tub looks brand new!!! You can pick it up at any Sally’s Beauty Supply. Get the powder bleach (I think it comes in small trial packages) and a peroxide to go with…maybe 20 vol. Follow the directions on the package for mixing. You will need rubber gloves. Test in a small area, leave on for a minute, then wipe off; it may need more time, play around with it. It should work on the rubber on your fridge, and the plastic on the stove. Don’t leave it on too long. After wiping it off, make sure to rinse any residue off with PLAIN water; I would rinse it a couple times just to be sure there is no residue remaining. With any chemical you use, you don’t want residue. Good luck.