Autumn is not an easy thing to model. Here are a few suggestions;
To blend the backdrop better with your scenery, make the painted grass a color that closely matches your ground foam.
Remove the red splotches from the yellow tree on the left. Try painting a wash of orange and light green onto its yellow foliage instead. Also, you might want to lightly mist brown spray paint to mute the bright colors. Rust, maroon, and brown are all realistic, muted fall colors.
Don't make the painted tree trunks in the backdrop so prominent or so brown. It will look better if they are gray and blend in with the background.
Add some evergreen trees to the backdrop and the scenery to break up the autumn colors.
The foliage on your backdrop trees is very well done.