Actually saturation looks nice, not all rock planets' atmospheres scatter blue like Earth's. But is it possible to move the cloud layer to an invisible sphere a bit higher than the ground texture, then make that cast a shadow to the surface? What about normal/bump maps, especially for cloud layer? In a 3D renderer it would be a lot easier to make lighting accurate for terrain elevations, so I would definitely be using that... AS it is, it looks a bit "flat".
On GIMP, I usually duplicate the cloud layer(s), then invert colours on the lower one, increase the size one or two per cent on the higher, and the reduce the opacity of the lower one with suitable layer mode so that it looks like a shadow from the clouds cast onto the surface. Then I might move the shadowing layer to some direction a few pixels if the lighting comes from sides or up or down... And I also usually bump the cloud layer at least once based on itself, which makes it gain some pseudo-depth in itself.
It looks good anyway.