The light/dark effect is used to create drama and realism in a composition. It helps you define the environment and the characters. The strongest outcome is achieved by using black and white, milder effects are produced by using an array of greys of different brilliance. Of course, the same light/dark effect could also be acquired by using colors.
Above, we have two geometrical compositions, one in black, white and gray, and one in white and three shades of blue, both showcase the light/dark contrast and is a great exercise to learn to shade.
Below, there are some more examples where I used other colors.

Hit the download button to get your file to start to practice the light/dark contrast saga.