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!
The warm palette colors were Cadmium Red-Orange and Phthalocyanine Blue. (Again, mine were M. Graham, and again, I used the 1/2 inch Escada Prado.
For this painting, the main trees were painted first using mixes of the two colors that were heavier with the orange than the blue. Other mixes, heavier with blue, were used for the background. But all of the mixes were done with the same two colors. Negative painting was used to create the trees and branches in the background, along with more of the scraping.