This is a very personal thing; wars have been started over less!
I always use red for the top layer and blue for the bottom.
Layers 2 and 3 are GND and power, and they're green and yellow respectively.
On a board with more layers, it gets more difficult. I keep the same colours for the power planes, because they're the same colours that I'd usually use to highlight the power and ground nets anyway. On a 6 layer board, the middle two layers are orange-ish and pink-ish. Beyond that, you're on your own.
I have in the past also been known to follow the same colour sequence as snooker balls: red, yellow, green, brown, blue, pink, and dark grey
