We are imagining a day out by a peaceful lake or slow stream. The water is calm, there are rocks, branches, leaf detritus and reflections. This may be a studio still-life but many similar components would also occur in a natural setting.
Make a line drawing, then look for a range of values to work in. Vine charcoal is quick and easy to change as you develop a line and value shape map. Consider this a preparatory sketch or make it the under-drawing for a pastel work. If you are working over the gray-scale with color don’t forget to fix first.
Now for color: Keep it simple at first with one key color, then develop form with tints, tones and shades. The key color is what you get a “hit” of right off the bat and can be different for each person depending on mood, light, intent etc. Once you have the key, then develop a harmony with the key in mind.
In the examples above see different harmonies: Red-orange key, red key with green complement and yellow-orange key with blue-violet complement.