The interesting thing about these formations—and all the other exposed basalt bluffs and cliffs around them—is that from a distance, like across the lake from them, they look BLACK. But the closer you get to them the more colors you see in the stone and the lichens that grow on them—reds, browns, ochre, yellow, even green. Before I started painting I studied every photo I'd taken that day, then googled for images of basalt, and finally decided I wanted to show both how black they can look, and how colorful. That meant I had to invent my own colors, which is always more complicated, but also more fun.
I really wanted to paint the formations as accurately as possible, so I used my digital projector to draw in the outlines, and the first painting I did was to establish the angular vertical shapes of the stone in both buttes with a color value underpainting. After the initial blocking in I ended up adding multiple layers of thin washes, including violet, gray, burnt orange, brown, and black, lightening and darkening alternately until it finally looked good to me. By far, the most difficult part was getting the highlights on the left butte to look right. It took me three days to finally hit on just the right hue and value of grayed red-ochre.
The bottom line is that I kept trying different things, and if I liked it, I left it, and if I didn't, I painted something else over it. I particularly appreciate how easy this is to do in acrylics. You don't have to use medium, although you can, and I used to—now I just thin the colors with water. It's taken me a couple years to learn (and remember) what you'll get with all different dilutions of washes, from barely visible to barely transparent. I do give them at least a full day, preferably more, to cure before I put on the gel medium isolation coat and protective layer, and when I brush that on, I do it with a very soft brush and very carefully.