How do I clean faux marble shower walls that has turned yellow?

You won't be able to get the yellow off with cleaner. I would paint over the whole thing or retile it.

To remove the yellow stains, all you need is some hydrogen peroxide and a clean cloth. Pour some hydrogen peroxide in a small bowl. Wear latex gloves to protect your hands. Dip a clean cloth or towel into the bowl of hydrogen peroxide. Wring out any excess liquid from the cloth or towel. The cloth or towel should be damp---not dripping with hydrogen peroxide. Wipe the yellow stain with the hydrogen peroxide-soaked cloth or towel. Do not rinse the surface or rub. Let the area air dry.

If the stains remain mix the following items in a bucket, adding them to the bucket in the order listed below:

8 cups hot water

¼ cup baking soda

¼ cup vinegar

½ cup ammonia

Allow the mixture to foam.

Use a soft cloth or sponge to apply the mixture to the stains.

Allow it to set for five minutes.

Rinse completely with cold water.

Hi Jenny, create a dense paste of more baking soda and a little bit of water. Apply the paste to the stain and leave for a few minutes before wiping it off. You may need to repeat until the stain vanishes. If it’s a tough stain try removing it with ammonia solution. Dilute the ammonia with water 1 part ammonia to 10 parts water. Test in a small spot first. Rinse the solution from your marble well to eliminate any residue, left from the cleaning ingredients on it and wear gloves.