Yes, the rack color is an attribute of the rack host (ie: the slot in the parent song, not the rack itself).
Even a binding wouldn't really help in this case because you'd need to bind back to the parent song and find the correct rack to bind to.
That said, the rack does internally have a reference back to the active rack host slot, so it would be possible to drive it from the rack - it then just becomes a question of what the UI would look like. Something inside the rack that controls it's color in the parent song. Perhaps the Rack Options window, but that's fairly disassociated from the rack state, and it should be state controllable. Hrm...
Seems like a useful thing - logged it.