The colors are more-or-less indications of which branch the Jedi is drawn to/affiliates with- Blue is for the Guardians, the warriors and combatants of the Order Green is associated with the Consulars, the most scholarly and pacifistic Yellow is the mark of the Sentinels, who blend the Force and the skills of the world to their own ends. Red has become affiliated with the Sith, although it has no bearing on the actual nature of the saber itself. A Jedi could just as easily be drawn to a red-bladed saber and be among the purest champions of the light. Mace Windu's saber was Violet because it used a fragment from a crystalline being that he came across during his younger days, and, due to his rashness, was responsible for the death of said being. He uses the crystal as the heart of his saber as a reminder to reign himself in.