Study of values on flat planes. Went with a more spontaneous approach and a loose sketch, refining shapes as I go, which was more fun and taught me how to manipulate the planes without destroying the "read". However, not sure if entirely successful overall because the shadow values seem messy. Also, not sure if adding highlights to guide the eye was useful.
Thanks to all those who generously share their stock on dA: fav.me/d3096xc
(cropped for purposes of study)
Photoshop CS6, Intuos 5 with art pen.
Edit: The dA thumbnail is a lot darker than the actual image.
Fullview for the correct values.