A thunderhead drifts across the African savanna, trailing a veil of purple rain. Cirrus clouds streak over a sunset, dyed hot pink and orange. Deep indigo storm clouds roll across the sky at dawn, edged with golden light and promising rain.
How do you paint that?
Everyone knows that a cloud is white. Or, when it's overcast, clouds are gray. But painting them is a different story.
A cloud at noon is not the same color as a cloud at 3 in the afternoon. At noon, the sun is at its highest point, and the light is blue. But in the morning or the afternoon, the sun is lower and its beams are more slanted, and thus more yellow. At dawn or sunset, the light is red, because the sunlight is filtered through so much of our atmosphere. Things like atmosphere ionization, during a storm, or smoke from a forest fire, also affects the color of sunlight.
When you plan your painting, decide what time of day it is. Usually when you plan a landscape, the clouds that come to mind are ones that you, as an artist, decide go well with that scene. So they may be thin and wispy, or heavy and brooding, or just big puffy cumulus in the background. Clouds give sky a wonderful texture, and they can be as far in the background, or as much in the foreground, as you want.
They impart mood. A series of small, puffy clouds give a lighter feeling than one massive thunderhead. Decide what mood you want your painting to give the viewer, then paint your clouds accordingly.
Rendering cloud form is also tricky. A cumulus cloud must look puffy, but each puff can't be the same as the last. It takes a lot of concentration to achieve randomness. Move your brush or pastel stick in small circles to render each puff.
Stay on the lookout for likenesses. It's no fun spending hours on a cloud, only to step back and discover that you have a wonderful portrait of Albert Einstein. It helps to bring in a friend or family member with fresh eyes. They will take one look at your cloud and tell you if it reminds them of anything.
Don't fall into the trap of using blue, gray, and white. Unless your picture is exactly at noon, the clouds won't be purely any of those colors. They reflect colors. Clouds over a desert will have reddish-brown undersides, while clouds over the ocean will have been blue or green undersides. Instead of white, try painting a cloud with pale yellow, pink and purple. Seen from a distance, all these colors together appear white. But it is an interesting white. There is depth, and it is not so flat.
In short, let the mood of your painting decide your color palette. That applies to clouds, too. Experiment with different colors. After all, white is so boring!