Here’s a 16x20 inch oil study of thunderheads on a warm July day.
I was amazed as I worked over a two hour period how fast the scene changed from minute to minute. Watch this time lapse video of a similar cloud formation boiling away. As soon as you have the shapes established, you have to paint the details from memory. But you can keep studying the scene for the overall color relationships.
The brightest whites and the sharpest details are reserved for the emerging billows at the top. The purer white colors of the closer clouds transition more toward warm pink or dull orange as the clouds go back in space. Light that has traveled farther has lost more of its cool wavelengths through scattering.
Whenever you paint these attention-grabbing "cumulus castellanus" thunderheads, look also for the shreds of old clouds sheared off by wind currents and dissolving back into the air. These often-overlooked “fractus” or “scud” clouds are the other side of the cloud’s life cycle of growth and decay. They lack the compact density of the billowing clouds, and are never as white.