Color Theory: How to Shade Correctly

How to shade shapes correctly is actually more complex than it seems. It isn't a simple case of dropping a light grey onto one side of a shape. 

How to shade objects correctly

The box at the top of the image is placed on a white background. To shade this box correctly you have to use the grey scale from white to black. First I have to shade the top, I have shaded the top  with a 20% grey. The top shade is the most important one, It provides the base for the rest of the box. So to shade face 3 I have to take into account face 1. To find the value of face 3 I have to find the halfway point between the value on face 1 (20%) and Black (100%) the halfway point between these value's is 60%. So face 3 is 60% grey. You find face 2's value by looking at face 1 and 3. Face 2 is halfway between the value of face 1 (20%) and face 3 (60%) so face 2's value is 40% grey. They all relate to each other.

The shadow of the object is halfway between the backgrounds value (white) and Black (100%) so it will be 50% grey.