I read something somewhere once, during the time in between TPM and AOTC, so we hadn't seen Mace's purple blade yet, theorizing that saber color was connected to Force orientation -

Red=Sith
Blue=Jedi
Green=Borderline, could tilt either way.

Which made sense for Luke in ROTJ as he was dancing on the border of turning to the Dark Side and for Qui-Gon in TPM since he seemed to look at the Force a little differently (Obi: "If only you would do as the Masters say, you would BE on the council", or words to that effect).