Actually, the colors do have some meaning, although most Jedi don't really acknowledge it after the Old Republic Era. Blue is the color of a Jedi Guardian, a warrior trained in the art of Lightsaber conflict and choosing to deal with most situations with his civilized weapon ignited. Green is the color of the Jedi Consular, who seeks to solve most problems through diplomacy and will only resort to combat if they deem necessary. Yellow is the color of the sentinel, who is a street smart Jedi that is somewhat of a middle ground that prefers to use personal knowledge and wit over all else. Violet (Purple) is the symbol for wisdom (slightly different from knowledge, think I it more like experience). Red used to be used by both sides, but nth Hedi shy away from using tear crystals as they are [b]synthetic[/b] (Man made). These crystals produce a stronger and hotter beam, and the Sith started using them to symbolize their strength over the Jedi. [spoiler]As a side note, red also symbolizes the Sith's [b]Control over the force,[/b] which is contrary to the Jedi's idea of [b]letting the force control them.[/b][/spoiler] So red doesn't occur naturally. Purple is an extremely rare color because t doesn't occur as frequently as the other three (Red, Blue, and Yellow) in nature. That is one of the reasons it is a symbol of longevity and wisdom. Jedi receive a crystal as part of their trials, and a colored gem speaks to them and guides them along their path. Although Jedi do not HAVE to keep that color, it is encouraged and many of the order keep true to their color, or alternate colors when they use two savers. An example of this might be Ahsoka's Green and Yellowish saber.

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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.

Theoretically, the colors are irrelevant. It's a matter of personal choice. Windu uses his because it reminds him of a past incident.