Concept art tutorial!
You need to make a few simple sketches, where:
1. The foreground is light, the middle ground is gray and the background is dark,
2. The same thing, but in reverse,
3. Try random colors. For example, the foreground is dark, the middle ground is light, and the background is gray.
4. Black in the middle and light around? Why not?
5. And etc.
First, try simple forms like in my example, and then try to complicate them by adding details and then even more details.
Here, I tried to summarize the exercise in one picture.