First off, a cast shadow is created by an object blocking the light source. Cast shadows are not just black, they have color and they are transparent, you can see through them. To paint a realistic cast shadow, remember these things: a shadow is more defined closer to the object creating it and it is also darker closer to the object. Conversely, the shadow is lighter and more diffuse the farther it is away from the object. To make a shadow as interesting as possible, include a variety of color, value and hard and soft edges. Your eyes and fans will appreciate this!