I have two, one is a few years old, the other is fairly new. Both are very white.
My understanding is that keyboards turn yellow when they are overly protected from sunlight and/or UV, or just kept in a dark room. For example, a piano kept in a dark room, or with the keyboard cover normally closed, will yellow. I’m not sure whether or not exposure to the Sun will reverse the yellowing, but I wouldn’t be surprised. In any case, if you expose your synth to a bright room with plenty of regular indirect sunshine (not direct because that’s bad for electronics), I think that should prevent the issue.
I have no idea if sunlight can undo the yellowing, but I wouldn’t be surprised if it does. Still, if you put your synth in a place with lots of frequent indirect sunlight, it should last longer.