Like in the center area of the layout, I started by painting the sky using three different shades of blue that I had left over from my N-scale P&CRR version 4. Unlike in the center section, this time I used plain white paint to paint some clouds. They are harder to see, but their subtlety makes them very effective, I think, in person. I lightly marked off the tops of the mountain ranges, and then painted the entire mountain range with acrylic "burnt umber", followed by "raw sienna". This made the mountain look fiery red; not what I wanted. I let it sit overnight and then the next morning realized that a light wash of gray might tone it down a bit. I did some experimenting, but that's what I settled on in the end. Some of those colors will come through, some will be completely painted over later. This is when I took the photo below of the backdrop on the left-hand side. I did the same thing on the right-hand side.
Next I used acrylic "hooker's green deep hue permanent" by Liquitex for the forests of the mountains. My intent was to use various shades of green, but in the end I only used this one color. I used the stippling technique using a 1/2" stiff, round bristle brush to create the trees in the bottom 4/5 of the mountains. The photo below is of the right-hand side of the layout at this point.
I then used the same technique using the smaller 1/4" brush to fill in the remaining top. The gray, and some of the burnt umber and raw sienna is visible through the trees. It gives the mountain a layered effect. The smaller stipples at the top of the mountain give the illusion that the mountain curves over the top. Each time I dipped the brush in paint, I stippled in a random pattern until the paint was mostly gone from the brush. The first 5 or 6 stipples are very dark, but they become lighter as there is less and less paint on the brush. With less paint on the brush, more of the background colors come through. That changes the color somewhat and causes the eye to believe that the mountain isn't entirely flat. The very top of the forest line needs to be just a little above the top of the mountain line, so that the sky color comes through the branches of the top row of trees. But those need to be very small to give the illusion of distance. The next photo shows the final result.
This is the completed right-hand side, which has a tall mountain going off of the layout.