Until last week I didn't even know these new keyboards could discolour... That is til I visited my parents and saw the iMac G4 that I helped them buy off eBay. It has a very yellow keyboard, that's the same type as came with the eMac.
Mine is very white, and hasn't seen much direct sun at all. If yours has turned yellow, I doubt there's any way to "reverse" it, as the plastic itself would be physically changed, rather than have any residue on it that could be "cleaned"...
If it really bugs you, you could source a "new" keyboard, as they (that old model) are pretty cheap these days.
