Windu walked the line between the light and dark sides of the force... his lighstaber form - Vaapad - is indicative of this: everybody else who learns it turns bad. He uses something akin to 'Force Crush' on General Grevious in the cartoon series, etc. and is still one of the most powerful Masters of the Light.
From a quasi-philosophical POV, we can see that there is no true existence of a 'dark' or 'light' side: Kreia hints at this in KOTOR II and Luke decides this in the conclusion of NJO... there are only dark and light motives. In the NJO series, a light-sider uses a scarlet lightsaber; in Revenge of the Sith, Anakin still uses his blue lightsaber (sure, he may not have had a chance to change it but the point remains).
In the NJO series of books (quite a few are poo, btw.) turquoise is also mentioned. I think that as far as colours go, there isn't any real limit: as long as the crystal fits the hilt and can focus the beam, it can be used... all colours should be randomly available in the game. The first crystal I even picked up was a silver one (KOTOR II) so they are out there - it just looked rubbish, like you were waving around a can of deoderant. I've had bronze and yellow (very similar: one has a darker hue than the other), light green (can't remember the name) and all the common ones.
To be honest, when it comes to the crunch, I'm not really fussed.
