And be SURE to immediately UN-CHECK the DEFAULT Cast Shadows and Receive Shadows boxes. GROUPS of Clouds look fairly bad when shadows are being cast on each other in very close proximity. The ONLY time to have the Cast Shadows ON is when there are small, independent clouds floating around and you want to see CLOUD SHADOWS on the ground (which can be quite realistic).

Also NOTE the LIGHTING Sliders for DIFFUSE and AMBIENT. These are usually your BEST ADJUSTMENTS when Clouds look Too Bright or Too Dark (other than what has already been mentioned). These are LOCAL Controls on each Cloud Surface. Your GLOBAL Controls are, of course, in the ATMOSPHERE EDITOR.