The Black Knight mechs are all lined up and showed against a dark background. The frame changes and its a light background and we see the Guren complete with the same float system that the Lancelot has. The Lancelot also appears right after Guren, in the same background. They are then shown together, yet not fighting, for several other scene changes.
The significance of the float systems being the same is that during the Black Knight portion of the OP, Gino's and Anya's mech can be seen with altered features and the Black Knight float systems, thereby lending credit to the fact that the different float system differences mean a switching of sides.
Kallen is also shown upset in her BK uniform in the beginning of the OP. In the ED a prominent RED KoR cape is on the floor near Nunnaly. Nunnaly isn't a KoR, but she is, currently, in association with someone whose character design uses red alot, and possesses the ability to become one.
The ONLY thing to suggest that Kallen isn't going to betray Lelouch is the fact that she is still wearing her BK pilot suit the second time we see her in the OP. Also, the low intelligence rating she got plus the 100% loyalty don't bode well for her I think.