Something beginners tend to forget or not observe is that clouds have shadows in them, they're not just pure white all over. Even clouds on a bright sunny day. But by shadow I don't mean black, I mean darker in tone.
The colors you use for this obviously depends on what you're using in your painting. My first choice for the darker tones would be white mixed with the blue you're using for the sky. Then if you need it to be darker still, for instance for dark rain clouds, add in a little of the darkest color you're using in the rest of the painting.
For instance, the paint-smeared object in my hand (Photo 4) is the moisture-retaining palette I use for acrylic paints. On it is Prussian blue, turquoise blue, raw umber, and white. In the clouds above the palette, I've used only blue and white, in a variety of tones. If I wanted to create a feeling of pending rain from the clouds, I'd use a little of the raw umber mixed with Prussian blue for a dark tone. Why raw umber? Well, because the clouds are part of a seascape and that's the color paint I had chosen for the rocks.
