Clouds

I find that clouds are one of the most difficult things for me to paint. Other people find it complicated to paint water or waves. Me, it's clouds. I have noticed that much of the time the bottom of a white cumulus cloud is gray and that gray is most often about the same value (lightness or darkness) of the blue sky around it. (Not the case with this painting on the left) I've made a mistake many times of making that gray area of the cloud darker than it should be. In other words, I were to take a photo of the clouds and convert it to black and white, the bottom of the cloud would often be the same shade as the sky around it.
No hard and fast rules with clouds though. It takes very clear thinking to paint them. They are far more complex than just putting a few strokes down on the canvas.