Take a look at this study by Stanislaw Zoladz. What Lovely, convincing light! Just one or two small triangles are enough to create a believable illusion. But what would the illusion be without the subtle mid values?
Here are a few images to experiment with. Paint the large, simple shapes very dark , or very light instead of middle value. If the shape you are about to paint is closer to dark than to light. paint it black. If it is closer to white than it is to black, leave it white. Read that last part again.