Darth Lucas said:

I think the yellow saber would be a good idea if it was like, the color lightsaber of a Jedi Master. Like when you reach that rank, you get a yellow blade. So, Yoda, Kit Fisto, Mace Windu, etc. would all have yellow lightsabers. But then again, it would be kind of confusing when Obi-Wan becomes a master and joins the council but still has a blue blade... oh well, it's a cool thought nonetheless.

I actually like the idea too.

The reason why Obi-Wan still has a blue saber could be because he has to be a senior member of the jedi council before he is given the same privileges as Yoda, Qui-gon etc. So the top ranked Jedi's would be Yoda, Qui-gon, Windu and formally Dooku, all with yellow sabers. The green colour saber can be preserved just for ROTJ where Luke constructs his own lightsaber, and since the jedi council is extinct, it does not matter which colour he chooses.

I dunno, these are just some ideas lol.