For distinction, I wouldn't be surprised if GL change dthe color of Obi Wan's balde to green for the "Greatest Duel of All Time" or whatever, but that doesn't run continually with his scenes with darth...
...Unless there's a third, DiFfErEnT lightsaber involved!!!!11one
I think it's just a coloring mistake and everyone is blowing it way out of porportion. It's a movie. Movies have errors. Nobody complains about ESB's potato asteroid field.