Malcolm,
I think the problem may be that your colors are too vivid for a backdrop. If you add some white or pale yellows to your basic colors you will get more of a feeling of distance.
You might want to try for colors that will complement the foliage you will be placing in front of the backdrop, but be a couple of shades lighter in tone (more white).
Also, reds are tough to control. You might want to add some yellow to give them more of an orange tone.
Another possibility would be to paint some distance hills with very light tones behind the trees off in the distance. Then, you can paint some smaller trees in front of them with colors just a little more intense than the hills.
Usually, anything painted on a backdrop, such as trees, look better smaller in size than the actual ones you plant in the foreground.
Jim