To lighten the undersides of spectral atmosphere clouds, you boost their internal ambient light slider. With that control and the density, opacity and other sliders, you have pretty complete control from white cotton ball clouds to dark storm clouds.  Plus altitude, height of layer, amount of fine detail and more.

And there is a spectral atmosphere high cloud type for use above your camera and a low cloud type you can fly your camera above or through or lower down around your mountain tops.

Cool idea for those seagulls Walther! One for the notebook.