Purpose:
To determine where values change when light moves over an uneven surface by using a range of values to paint the different values in a realistic, monochromatic self-portrait;
To effectively communicate an image that looks like you through careful examination of the values and structure of your face.
In order to paint facial features, you need to have a good value range and be able to visualize the colors and create a good range of values. To be able to create a realistic face with only shades a skill that is necessary is being able to create the correct colors/shades to match the real world.