Look at Me!
It is easy to imagine that George Lucas planned every aspect of the saga decades in advance of actual production of any given chapter. That is until you begin to read more and more about how each film went from core concept and rough screenplay to the final edited film. Han Solo was once to have been a hideous bipedal amphibian, Darth Vader was to have a holiday home on a lava planet where he would forlornly feed wild gargoyles like he was an old man tossing crumbs to pigeons in the park, and Yoda looked suspiciously like the Travelocity Roaming Gnome!
And in that same vein, Mace Windu was not originally intended to have a purple lightsaber. The clearest indication of this is the "Sneak Preview" action figure of Mace that was produced before the release of The Phantom Menace, as well as the one produced after the film hit theatres. In both cases the Jedi Master's lightsaber accessory had a blue blade. After Episode 2, the more familiar purple blade began to appear in toys because the color of his saber's blade had been revealed at last.
From a production design standpoint the new color was introduced at Samuel L. Jackson's bequest. Knowing he would appear in crowded scenes filled with lightsaber wielding Jedi Knights, Jackson wanted to stand out on screen. To that end he requested the unique blade type which certainly makes it obvious where Mace is should he be in any given shot. While this explains the origins of the blade from a behind the scenes standpoint, it does not do so within the context of the storyline itself.
There Can be Only One
To explain the presence of the purple lightsaber I found my answers within what was presented on screen. While at first glance the venerable Yoda would seem to sit at the had of the Jedi Council. However, if you watch closely there is strong evidence that despite his stature in the Jedi Order, he is not in fact their leader. That honor goes to none other than Mace Windu (rather appropriate since he was the first character named by Lucas, then as Mace Windy, when he began to develop what would become Star Wars).
When I have shared this observation with Star Wars fans I have been met with raised eyebrows or confused looks. But upon closer examination it is easy to see how I arrived at this conclusion. In Episode 1 for example, Mace dominates the Jedi Council meeting on the fate of Anakin Skywalker. Even the great Yoda defers to Windu as if he is a subordinate. Prior to Episode 1 it would have been an easy thing to imagine that Yoda was the most powerful and respected member of the Jedi Order. He was, after all, one of two survivors who still eluded Vader and the Empire. For that reason I believe it is difficult for most to look upon Mace Windu as the head of the council, but the evidence is certainly there. Windu has a commanding presence in the Council chambers in all of the prequels, usually having the last word in all decisions.
This is my justification for Mace Windu’s lightsaber. Again, I am not steeped in the non-film history of the lightsaber or the crystals involved in their production. I have not researched what others have written on the significance of purple versus red, blue, or green blades. Nor do I care to. The lightsaber justification is the one that tends to be the most hotly debated by those who have read a tremendous amount of the Expanded Universe material. And so I reiterate that this, like all of my justifications, is based solely on what is presented on screen.