Yesterday I did a little experiment to see if I could retrobright my hard top grab handles back to their original grey color. Yellowing is caused by Bromine in the plastic/rubber breaking down over time.
It seemed to work well. I used a 12% hydrogen peroxide hair cream bleaching product. Masked off the affected area, applied the product, covered with plastic wrap, and then exposed it to UV under 2 black lights for 4 hours. Will rinse and repeat for the second handle.