Once you start painting larger WC sheets, you need a lot of pigment and washes. I have seen several artists use white ceramic plates. UK artist Joe Dowden used ceramic plates and influenced me. I purchased several white ceramic serving dishes (Amazon) that were rectangles. The fit on my table better than the round plates.

oh, I'm an addict. Here's one - early John Hurtley in a custom color... and a bonus landscape of Northern California, Sonoma Coast.

That’s an interesting approach; I’ve never tried it. I usually try to work out a balanced composition using sketches beforehand, and I often create color sketches afterwards as well; that helps me visualize the individual steps of the process. It’s not as intuitive as your method, though, so I’m going to give your approach a try.