You can retrobrite it.
This is what a lot of folks will do. Be warned - it can destroy a case, too. I have a C128D keyboard that became horribly marbled by the process (aka ruined).
But it can also make an yellow/brown machine look almost brand new. From what I've heard, however, it isn't permanent. One friend of mine who has done the process many times has had one of his retrobrighted machines - down in his basement away from UV light - turn back to yellow in a year's time.
It's gotten to the point where he now asks folks on Ebay if they retrobrighted something he might want to buy, because he knows it's potentially short-lived.
I'll still do it, though, rather than keep something bright orange/yellow. But if it's just a little bit turned? I can live with that. Usually. (Or I happily use it while secretly planning to upgrade that case/part at some point in the future.) That's how collections grow.