I think the colours for the back trees are fine, instead I'd focus on making them look more like trees. They're too textured, and they're all so straight and evenly spaced that they look man-made. I think if you get some more organic thunk shapes and variation in there, they'll look great even being solid black.
If you meant you want to do a middle layer of trees, make them black with small dark brown highlights, perhaps? Again, the key is their shapes, not their colours.
I think the back grass looks fine. Normally I'd suggest relying more on clusters than lines, but the style of this entire image seems to be a line-heavy, detail-overload one with black everywhere, so flattening the grass would just make it look out of place.