How to Clean a Yellowing Fiberglass Tub Fiberglass bathtubs may start to turn a yellowish color as a result of soap scum or hard water. These stains can be very difficult to remove. However, a mixture of cream of tartar and hydrogen peroxide will easily remove any stain and restore the tub to its original white color. ** I'm going to try this on my ugly yellowed tub and then submit the results. ** Results - The tub is not white but looks a little bit better.