In a response to an earlier comment, I mentioned the concept of "maniera lavata" - an Italian term for what is often called "underpainting."
On my previous layout I tried painting the colorful fall trees first, but the resulting trees lacked depth. And when I'd try adding the darker colors as shadows on top of the colors the whole thing often got away from me. Too many colors on top of other colors resulting in a dark, dead, lifeless, mess. Lighter colors applied over the darker underpainting really helps solve his problem.
The other issue I've had in the past is getting the tonal values and colors somewhat consistent across the backdrop. A solution to this issue is to pick a color palette and stick with it throughout. I'm using eight colors on the backdrop - which seems like a lot, but that includes a couple of grays, two greens, and the base scenery and sky colors. Every color is mixed from some combination of those eight basic colors.
Another key to avoiding an inconsistent look to the backdrop leads us to our second fancy Italian art term in this blog post, "Alla Prima." This translates to "at first attempt." It indicates an approach where a picture is completed by painting on the entire surface of the canvas all at one time, instead of fully completing a specific section, say a corner, of a painting before moving on to the next corner. Alla Prima results in pictures from the Impressionist school of painters. Okay, enough art lesson for today.
In this case I'm not actually completing the entire painting in one step, so it's not truly alla prima, but I am completing all the underpainting, highlighting , etc.. of the backdrop one wall at a time, trying to keep the entire thing somewhat "loose."