The different balls of the Wizard's Rainbow correspond to the thirteen colors of the window. And Black 13 is the most powerful of those, controlling the others, like the lord of the rings. Also, there's a parallel to the Dark Tower itself, sitting at the center of the twelve beams, with their twelve guardians. So I think the black innermost window is objectively innermost, rather than just appearing that way from a particular viewpoint. Though I also doubt the others are concentric; they'd probably just somehow ring around that one, basically side by side. How it works, exactly, I don't know, but that is the symbolism, I believe.