Modeling solid objects is at the heart of the central mystery of representational art—how light and dark marks on a two dimensional surface can convincingly portray three dimensions.
Rectangular Objects such as buildings or boxes are generally a light value of their local color on the lit side and a darker value of the same color on the unlit side. However, the unlit plane is darker and cooler at the edge where it meets the lit plane, and lighter and warmer elsewhere. This “plane change accent” is often a very subtle effect, but it is worth exaggerating in your paintings. It makes the light look brighter and heightens contrast where you usually want it, at form edges.
Round or cylindrical objects such as balls, poles, and mountains also exhibit a plane change accent, called the “core of the shadow.” It is most easily seen in a strong side lighting situation. Since the plane change is gradual, the shadow is soft-edged.
Unlit Planes have color variations due to reflected light. For example, the unlit side of a gray skyscraper is usually bluish at the top where it picks up light from the sky, and brownish at the bottom where it picks up light bounced off the ground and surrounding buildings. Likewise, there is often reflected warm light under eaves and on porch ceilings.
Cast Shadows outdoors are usually darker and cooler than the unlit side of the object that casts the shadow. Cast shadows are not just black or blue. Observe carefully and you’ll see that they have color. For example, car shadows on a sunny day are warm brown/gray under the car where they are reflecting the chassis, and a cooler blue/gray beside the car where they are reflecting the sky. Finally, cast shadows are darker and harder edged near the object casting them. Further from the object, shadows usually get lighter and softer of edge.
Rendering Exercise. Paint a white cube, a ball, and a cylinder on a light table top with strong side light and a dark background. Experiment with differing light sources. Put different colored papers under and alongside the shapes and see what happens to the color and value in the shadows.