It isn't always easy to understand how value (light and dark) works on a 3D surface, especially if that surface is a person. For our plane portrait project, I tried to help students learn how to use ratios to draw the basic features of a face before breaking it down into smaller, flat surfaces. Basically, students were creating their own paint by number type of drawing and value shifts would come with shape shifts. It was met with various degrees of success, which isn't surprising considering the enormous range of visual and cognitive abilities of my students.
|student divides pencil drawings into planes|
|student paints individual shapes on face|
Some of my students needed to work in Wiki Stix, which I then hot glued their lines for them, while removing the waxy wicks. That way they could paint in the tactile borders. I would mix values of paint if they didn't have any vision. One student had enough vision to draw and mix independently. He didn't get the planes as sharp or "robotic" as I'd hoped, but it was the best thing he'd ever painted, and it's of Stan Lee, so that's a good thing.