Looking on the layers panel, you can totally see that it is an organized mess. I used these layers in a way so that the bottom-most layers contains background elements, and the elements in the foreground are at the top-most layers. This is standard practice for creating landscapes like these. I used purple, blue, yellow, and pink for the gradient sky, because I thought it looks nice that way, and then I added in clouds with translucent opacity, and drew shadows with an opaque opacity. I added in mountains, and some mists between them, and the three cliffs. In between these masses, I added in these almost-transparent gradient layers to make these mountains look better. I also used the clipping mask to add in the shadows of the pine trees, and the grass. Rather than just changing the value of the shadow or highlight colors for the trees in the piece, I usually increase the saturation, and decrease the value for shadow colors, and decrease the saturation (a little bit), and increase the value of highlight colors. This is not only just to make them look better, it's just to prevent the black look of the shadow colors, and the obnoxiously bright highlight colors.