I did like the idea that worked until AOTC which was blue = apprentice, green = master and red = Sith. White = training fits, and I personally don't mind Windu's purple sabre, especially when Dooku has a gold one in MagnoliaFan's AOTC, because they denote Windu's near-darkside fighting style (purple = red + blue) and Dooku's ambiguous character (yellow = red + green).

All-blue until ROTJ also works, but my preference is for six colours:

White = training.

Blue = Knight.

Green = Master.

Red = Sith.

Yellow = ambiguous or maverick characters, such as Dooku and potentially Qui-Gon.

Purple = badasses (Windu) — especially works if Windu is also portrayed as a maverick and/or overly suspicious of Anakin (someone raised a parallel to Snape in Harry Potter). The only other character I think could pull off a purple one is the Emperor (if he should have one at all); that means that we get a purple-on-purple duel and it could mean purple comes across as a "boss" colour but it reinforces the "not so different" aspect.