The elements and principles of design mentions value, and contrast, but doesn't combine them as they should be considered as value contrast. In regards to the painting, if you combine dark values with light values as seen in the blue marble - you gain the illusion of shape and form. Where they are placed and how quickly they fade from one to the other has implications that reveal whether a shape is concave or convex and to just what degree. Focus first on establishing the relationship between light and dark values to convey what sort of shape you want. Stay tuned for the next concept I teach to my students, color intensity.