InfoDroid said:
Just one more sidenote about Council members having puple lightsabers. To rationalize this idea, it may make sense in a battlefield situation for other Jedi to distinguish who is "leading the charge" in a potentially confusing situation. The downside to this would be the enemy could also easily pick them out.

Regardless, I have no problem with Dooku, Mace and Yoda or any other council member or "Advanced Jedi" having purple lightsabers. Akwat Kbrana is right, it only serves to strengthen the Dooku element of the story. It's a testament to how high a level he had attained in the order.

What about Ki-Adi Mundi? We see him with a lightsaber in ROTS. Maybe his should be purple too?

Making purple a sign of rank would seem confusing. A whole lot of the Jedi action we see are indeed masters on the council, so purple would not stand out that much. Also, would Obi get a purple blade when he achieves that rank?