Quote Originally Posted by Xurts
Just because he is seen absorbing a blue materia doesn't mean that is the only one he used. I'm sure he had a red one in his arm too, we just didn't see him put it in. What is so outrageous about that? We don't need to see everything that goes on behind the scenes.
Sadly, this is not correct. You see what color of Materia is active at a specific time, as shown with Yazoo and Loz near the end. We see that Kadaj has a blue/green (I thought it was green, some say blue, so whatever) glow on his arm, implying that materia color is being used. Therefore, he is using that color materia to summon Bahamut Sin