Transcript:
Huey: You know what's great about Sam Jackson having a purple lightsaber? It's such a "black" color. Only a black man would have a purple light saber. I mean, it's kinda like grape soda. Caesar: I told you before, Huey...they don't all have to be deep.
Canon explanation: Mace Windu has an unusual mastery of the force that treads dangerously close to the dark side. His distinctive lightsaber color (Sith Red plus Jedi Blue equals purple) is indicative of that.
Real-life explanation: Lucas asked Jackson what color lightsaber he wanted, and Jackson asked for purple. Lucas went along with it.