Here is the first layer of greys for the tree and landscape.
I used Prismacolor markers for this first phase, as the areas are so vast and I don't want to burn out my most precious markers. The colors I have chosen, in case you were curious are: Parchment for the bark, Warm Grey 10% for the leaves (although maybe a Cold grey would have been a wiser choice..let's see how it goes), Cool Grey 20% for the sky (I'll add heavy clouds an then highlight them with pastels). I'll use some earthy tones for the underground portion and the serpent and some cooler shades for the water maidens, the trees and the mountains.
I'll keep you posted!