In his book "Color in Sketching and Rendering" Arthur Guptill recommends painting simple objects with black watercolor to study the effects of light.
Start with uniformly colored objects set up in window light. Then, later, you can put the same objects outdoors in the sun "to acquaint you with the vitality of outdoor light."
In this demonstration example, he sets up a simple paperboard box with light coming from the left. One of the first questions to ask is which is the lightest plane (E) and which is the darkest (A and F). You can hold up a piece of cardboard of the same color next to the object and turn it in various angles to the light to see how the tones change to match those of the subject.
Even a single plane can vary a lot in tone. He notes that (a) is darker than (b) because of the falloff of the reflected light coming from the right.
After doing a few of these studies, it will be much easier to paint a more complex or dynamic subject, such as a building, a figure, a portrait, or an animal.
From Arthur Guptill's book Color in Sketching and Rendering.
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