The cool palette was made of Phthalocyanine Green, Ultramarine Blue and Ultramarine Violet. These colors are considered cool because they are all in the blue range, even the green and violet (warm greens would tend more to yellow and warm blue and violets would tend more to reds).
In my case, I used the M. Graham brand of these colors. A half-inch flat (I used an Escada Prado) was used to paint the background, leaving the main trees the white of the paper (Strathmore Aquarius II). Paint was applied thickly and allowed to dry somewhat, and then a palette knife was used to scratch out the trees in the background. If the paint is too watery, it just flows back into the scratched out area, so the waiting is important, but you need to scratch before it gets too dry, as well, or the scrapes won't take. Practice, practice, practice!