This illustration by Wesley Allsbrook for the New York Times is a great example of creating depth through  foreground, middleground, and a background.  Notice how the elements of the composition, (the bird beaks and the leaves pointing,) guide you to the yellow green middle with the man emphasizing the story of birdwatching. The foreground has more defined detail and colors are more pure and clear with both strong shadow and highlight elements such as the light on the red cardinal's feet and the highlights in the eyes. The middle ground with the man has much less detail, color is faded, and shapes are smaller. In the background shapes move toward abstraction because shapes are much smaller and therefore shapes lose detail altogether and the colors, including shadows, are far more faded and the highlights have a smaller value change from the mid-tones.The source link is below.