Blue is for a Jedi Guardian,a master of lightsaber combat. Green for a Jedo Counsular, those who seek to understand the force Yellow for a Jedi Sentinel, those who mix the two. Red is the traditional color of sith, those who seek the death if jedi.those new to the sith or unable to make any(Grevious) may use old jedi lightsabers or trophies of dead jedi. Purple is somewhat of a neutral color, in Kotor several dark jedi wield then. I would say anything outside whats listed above us a neutral color and simply color preference.