Below left you can see that the distant mountains are getting their second layer of lighter color over the very dark green/black. This alone could suffice for that particular area, but usually some highlights are added where the light falls. To the right, you see a lone tree having its individual leaves painstakingly painted in with a liner brush. The paint needs to be thin and opaque in order to effectively depict the foliage. Notice the dark gaps in the branches that give the tree a realistic feel.
And here is a final image of an exquisitly painted landscape by Smith. Though each leaf and blade of grass looks individually created, some of that effect is an illusion done with applied layers. Nevertheless, this kind of painting takes a lot of time if done right.