I believe it's impossible for any part of a cloud to be darker than the sky, at least in a traditional setting.
Also the darkest shade should be lighter because the cloud is thinner there, and so more light passes through/is reflected.
yep on a bright sunny sky and in the same distance clouds are in terms of value always lighter, or in the shadowy parts about the same value, but with less saturation - which makes them less intense.
Howeve rsky tends to get brighter to the horizon (atmospheric perspective or blueshifting) - which also affects the value of the skay and the clouds
clouds much closer to the viewer therefore can have a lot more contrast, as the already faded out sky in the farther background has.
Stormclouds are usually always darker than the sky.
