Ah ok, that wasn't how I saw the scene as being set up, sorry. In which case I think that falls under the fact that it "does not work with non-monotonic operators", of which Saturation would be one by the look of it.
If you want to use something as a background in the render though, why not just render with alpha, and composite in post? It would be more tricky if it was an object, but for a background that might be the easiest way to go?