Pretty sure the "canonical" meaning is because Windu's fighting style skirts across the edge of the Dark Side without falling in, and his lightsaber is a mix of red and blue to represent it. Idk if that was retconned with the EU though lmao

Purple has been hotly debated, especially since it wasn't part of any vision of the force and was literally retconned in by the actor. It's like the Vulcan Neck Pinch of Star Wars.