I'm sure differing Lightsaber colors originally came about to make dueling scenes easier to follow, and keep the guys doing the rotoscope FX from going insane. Now, it's probably a couple mouse clicks and done. ;)
Vader's was actually blue in McQuarrie's early conception.
In universe, I would think a Lightsaber color is purely a personal preference. There has to be a switch on the hilt for that somewhere?