You can change your saber color in game. Some colors depend on which path you take... As a Jedi knight making evil choices you get access to red as a color, ect. Or you can change it with crafted/dropped items. The pre order crystal just let's you change it earlier and easier if that makes sense.
That's one thing I was wondering about.
A Jedi going full dark... can they defect to the Empire and vice versa?
I doubt it but it could be really fun.