With most of my environment concepts I take in textures from photo references and use a cloud brush, as I learned, because the point is to give an idea as quickly as possible. However, I have been studying clouds a bit, and I was inspired to try painting them without the cloud brush or textures.
When I had tried environment concepts for the first time I went for the soft round brush. I did that for painting trials, too, of characters. I always go for it, and I think I have finally learned it is not really the best choice. I've really been enjoying working with the hard round on low opacity (bottom left), but the oil (bottom right) was a lot of fun, as well. It might also be that I simply got better as I went through the exercise!
Below is a photo I took of the clouds on a particularly beautiful day. If my camera hadn't died, I would've come home with fifty cloud photos on my camera! I looked at these images to guide colour choices and shapes.