Darth Venal said:

No offence, but the change of light sabre colour, for the vast majority of people, is completely pointless and conveys absolutely no new information.

Semiotically, it conveys a whole lot. Conventions of Star Wars aside (red for bad guys) red has connotations with blood, searing heat etc.

Giving him a colour other than red conveys a lot in itself. We ALL associate a red saber with the bad guys. Him having something different from both the good and the bad makes us uncertain about his character.

That, and the aformentioned and already demonstrated editing to make him seem like a better person. Here are a couple of my rough editing ideas in this respect:

http://www.youtube.com/watch?v=onooMQABtcg

http://www.youtube.com/watch?v=AsJ0Dlx1hJM

I think purple would be a good choice, but not the Pinky Purple of Mace's. That way it's different from the all-blue of the current Jedi order (assuming we're talking about an edit removing all green sabers), but doesn't take away from the impact of Luke's Green saber in ROTJ.

This way, having a purple saber in the prequels actually serves a purpose other than Sam Jackson just asking for one.