I got the original idea from here. However, I changed a few things up a bit.
First, we made ours landscape; it seemed to allow for more trees. Next, we used colored squares of tissue paper and collaged them on. This allowed for some process of color-mixing since we were overlapping the squares. Finally, we left the snow bank white, and added salt to the water-colored sky to look like snowflakes. My 3rd-4th graders really liked this project.