I agree with keeping Luke's ROTJ saber green. Partly for the original reason, and also because it seems to be used in the PT as a sign of a superior Jedi — Qui-Gon and Yoda have them, for example. As for limiting the number of colours, I don't think that's much of a concern; one thing I really liked about MagnoliaFan's edit of Episode II was that Christopher Lee's lightsaber was recoloured to yellow, which (combined with removing his scene with Darth Sidious and any references to his "master") recasts him as a much more mysterious, ambivalent character. I'd really like to see the same changes made when Adywan gets around to editing that film.