And is there really some official rule about only Sith having red lightsabers? Is there any reason why a non-evil Jedi couldn't have one? I mean, Mace Windu's is purple.
It has something to do with the way they focus their powers through the crystal. Reds and Purples are better at focusing the Dark Side of the force, while the others the Light Side.
Most of that knowledge comes from the games and novels though, I don't think I 've ever come across an official reference to why the Sith choose red.