The years passing on make many types of 'white plastic' become a kind of yellow,
and if the sun (also tobacco smoke) has reached the keyboard, the color will change faster.
Does everything works as it should, and the color is just a cosmetic issue?
If so, I think the saying 'If it ain't broke, don't fix it' is a very good advise.
And thinking about it, white looks like plastic, a bit yellow'ish make it look like ivory.