When painting clouds using watercolor, the white of the clouds will be the white of the paper. Don't stress about trying to paint around the shapes of the clouds, but create them by lifting off paint using something absorbent, such as a piece of paper towel or corner of a clean rag. If you find the paint dries before you've time to lift off the clouds, try first painting the area with some clean water, so when you apply the blue you're working wet on wet.
Start by selecting a blue, mixing up more than you think you'll need, and painting it across the whole area with a broad brush. Don't fuss overly about getting it a completely even wash as once you start lifting off paint to create the clouds, you'll have variations in the blue anyway.
The test sheet shown in the photo was painted by Greenhome, who says: "Before embarking on this [painting] journey, I thought a cloud is a cloud is a cloud. Not so any more. I find myself scrutinizing clouds quite obsessively these days. I did this test sheet with five different kinds of blue (cobalt, Winsor, cerulean, Prussian and ultramarine) and two different cloud lifting tools (scrunched up toilet tissue and a small sea sponge).
As you can see, different blues give the sky quite a different feel. Select a blue that fits the scene and the location. The sky definitely isn't always the same blue.
Once you're comfortable with this painting technique, start adding more color into the cloud area for shadows within the clouds. I like using Payne's grey for dark rain clouds, but experiment with adding a little dark red to the blue to create a purple-tinged shadow.