Here are a couple of more preliminary watercolor sketches of cloud formations. I photographed this cloud bank late last month. I was struck by the smattering of blue sky between the solid mass of white-edged clouds in the background, and smaller steel gray clouds hovering in the foreground.
In the first sketch, I laid down the blue (cerulean deep) first, and painted in the clouds afterwards. I used lamp black for the massive cloud bank and charcoal gray for the foreground clouds.
In the second sketch, I drew the outlines of the cloud edges, and some of the borders of the internal tonal variations before using the wet-on-wet technique to build the clouds. I dabbed in a lighter blue for the sky area between the clouds.
Adding the lighter blue after rendering the clouds was better technique.
Here is the reference photo: