You can "duo" it, too. Use this tutorial ("The Claw") to separate out a layer of highlight tones (user defined), and a layer of shadow tones. Then I just do a saturation/colorize on each of them, one warm, one cool, and the midtones (now your background layer, or what shows through of it) stay neutral and provide a transition.
There's probably 101 easier ways to get a similar look, but that's how I do it. "The Claw" is powerful.