I am currently having a difficult time understanding what the problem is. The clouds in the slab have different depths at different areas. In some areas light travels through a lot less stuff than it does in other areas. The denser areas naturally occlude more passing light rays than the thinner areas, so the darker areas are surely thicker clouds. Also when considering the behavior of sunlight, sometimes light from the sun strikes clouds in unusual places.
Some things to consider…
Look at the Skylab settings. As a general rule the Sky ambience needs to be fully white. When this color is fully white it allows materials to glow at full force. When you add colors other than white to that channel what you are doing is lowering the total amount of “ambient” light available for your materials. That being said, if your sky ambience is black then your materials wont glow no matter how high you set the material ambience slider.
Looking at your screen shots I can see that the Ambience slider is at 100 yet there are still areas of the cloud that are somewhat darker. Ambience can be used to bake away that darkness but it comes at a price.
I think you should do the following.
1. Make sure sky ambience is fully white.
2. In the material lab turn off all Diffuse. Increase Ambience to 100.
3. What you should have now is a uniform cloud that doesnt have any highlights or shaded regions. It should be a gray somewhat flat looking cloud.
4. Add in diffuse in small doses until you get a little bit of highlight.
Now, if for some reason thge steps above dont work then try to play with the shading mode. Off hand I cannot remember what the sequence is, but most clouds are set to Basic Shading. Most likely that will fix it for ya.
Best of luck, please report back with your progress.