Well that's certainly not normal. Anything you could have done to your Macbook? Perhaps you handled it on the sides with something on your hands to discolor it? Do you carry it around in a case, and if so, have you checked inside the case to see if that's the cause?

How old is your Macbook and how long has it has this coloring? Is it a black Macbook or white (with purple)?

Take it by your local Apple store - maybe they'll know...