The fiberglass shower in my house has also turned yellow, my guess is painting is the only thing that will fix it. You will find many companies that refinish tubs and showers and they all spray paint.
White plastic usually yellows due to the Bromine added to it during manufacturing. It can be whitened using hydrogen peroxide. It needs to stay on for a while so a gel works best. Below is a link that uses hair bleaching material. How to Whiten Your Yellowed Plastic Gadgets - HardwareZone.com.sg
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