What a lot of us do is to put a layer of 50% gray (rgb 128, 128, 12 on the very top layer then set this layer to overlay (effectively making it invisible). Then use the dodge/burn tools to add shades and tints to the gray and what this does is shades or tints the terrain. You could use straight black and white paint instead of dodge/burn. To use the technique with what you have here, you might need to blur the boundaries between the color shifts. It's very much of a time-killer and it can also end up looking more painted but I always like it What you have is nice and I think you're right on the verge of something really cool.