From a recent Watercolor Sketchbooks class with Judy LaBrasca:

"On Tuesday we did drawings by sense of touch without looking (object in a bag), then drawing it looking at it (out of the bag), and painting it in 3 color glazes. Here are some examples (the object is a fluorite crystal). The assignment is a “dos a dos” book (2 books in one), draw an object or scene in 2 lights, for example: “night and day” or “natural light and artificial light”, or “light from one side and then the other side”, etc. hint: look at shapes of shadows and highlights."