I am reading a watercolor book by watercolorist Robert Wade. He recommends practicing pages and pages of cloud/sky studies. Here is my first. Using a wet on wet technique in the cloud area. Ultramarine blue and a mix of ultramarine blue and quinacridone sienna for the dark area. This only took 5 minutes total. Of course there was a wait time between letting the clouds completely dry before putting in the hills in the foreground.