Technical Support wrote:
Yellowing is typically a chemical reaction that may not be reversible. However, you may try something like Woolite to see if it restores the white color.
Even if some of the light is normally unaffected by the yellow shell color, it kind of "melds" together with affected light. Given this, I don't think it is a problem to gel all of the light, just a smaller amount of correction should be used.
Thank you again for all your assistance over the years, and for this helpful suggestion. I will try woolite first, and then possibly oxiclean later.
thank you again.