When you start your backdrop painting, remember that you don't always need photo realism for rural or mountainous scenery. Often just a rough shape of the mountain in an appropriate color palette will be sufficient. Closer mountains will have colors that appear similar to your layout's scenery. However, keep in mind that distant mountains on the prototype appear in progressively bluer shades until the mountain is just a "purple mountain majesty" in the distance, so you don't necessarily want to use the same colors as your foreground scenery.