The wrist-rest and trackpad area gets dirty over time. I suggest using Apple Peelz: it’s basically a thin, clear film that protects the covered areas from dirt, discoloration, and minor scratches.
I suggest getting two “Wristguardz” and one “Trackpadz” (which actually comes with two sets) — you’ll need extras because you’re likely to screw up the first application, ripping the film or getting too much dust under it (like I did).
Every 6-9 months, they’ll start turning brown from dirt and skin oils. Without the film, all of that would be permanent discoloration on your laptop. But if you’ve had the Apple Peelz on the area, just peel it off and stick on a new one.
(This is not a paid advertisement. I’ll make no money if you go and buy 500 of the things because of this post. I just used them myself and liked them.)