Oddly enough, if you count all possible color/anti-color pairs, you will get the number 9. But brainwave is still right, there are only 8 gluons. The reason for this is somewhat obscure, buried as it is in the intricacies of group theory, and it ends up sounding like a rather Tolkeinesque myth. It turns out that if all nine gluons existed then quarks would not be subject to confinement, and free color charge would be everywhere and the world would be completely dominated by the strong nuclear force, rather than the electromagnetic force as it is now. The absence of one combination forces quarks to be confined, and hence the strong force to have a very limited range.

Unfortunately, explaining which one is missing is kind of complicated. It is easiest to talk about gluons as if they were straight color/anti-color pairs like rb* (red/anti-blue) or gg* (green/anti-green). But it seems that the physical gluons are actually combinations of these such as (rb* + br*)/sqrt(2). The ninth, "missing", gluon is (rr* + bb* + gg*)/sqrt(3).