The leaves will be brightly colored, and will affect the intensity required to accent the trunks, so I will complete the leaves next, and determine at that time whether to leave the trunks light or to darken them.
I started with a Cream colored pencil base for all of the leaf areas, which includes almost all of the board that still remains white. This step is fairly easy because you don't have to keep looking at the reference photo for guidance. You do have to be careful to not accidentally get this color on the aspen trunks. Where the cream touches the trunk, carefully erase any of the graphite pencil guide lines that may be left.
In large open areas, I used vertical strokes to fill in the base color; however, there are many small leaf areas that are surrounded by tree branches that would be very difficult to fill in using vertical strokes. Instead, I drew in the lines in those areas in the direction of the branches.
Some of the branches on the lower right side of the photo have a slight white highlight on the tops and right sides of them. I left those areas white. With colored pencils, the white is rarely used. It's best to use uncolored white paper to be the white in your illustration.
When I filled in the leaf areas adjacent to the dark blue sky, some of the blue bled into the cream color. This is not an effect we want in the finished drawing, but it is not a problem here, because there will be several more layers of oranges and yellows on top of the cream, so the blue will not show up eventually.