I fill in shape 3 first. At this point, I am not concerned with what parts of this field are in light or in shadow. Instead, I use this one green to fill in the whole area. The color mixture I use is neither the lightest green in that area of the scene, nor the darkest.

Different artists painting the same scene would probably come up with different interpretations of what the average color of a shape might be. What we are really doing when we paint an average color is creating a color mixture to act as a shape's color representative. The most important thing is to make sure that whatever average colors we use to fill in our shapes, they have the correct value and color relationships to one another...