I have not taken an Albook apart, but I would think that this i colour in the plastic. They mix colour with the plastic prior to the injection moulding so that the keys are solid grey...all the way through.
If you have a look at the letters....well, I have a pb667DVI, and the letters are printed on. no sign of wear so far, and I have not seen any other books with this problem...
The keys on the new books, would have to be just a cutout in the key, to let light shine through.
bottom line: no problem. letters and colour will not wear off...