To make the forest, you could use the dupplication tool once you have a tree with color and either select "behind" in the option to paint it "behind" or use it normally when you want it to be in front of it. But it need to be opaque to do that.

In my opinion, the change of color is great if you have a decidious forest in autumn, else not so much. Here, I think the effect is too strong.

Great mountains by the way.