Take a look at the black box below. Although the entire box is black, each side is a different shade of black. This is because the light is hitting each plane (or side) differently. Without light, we would not be able to differentiate between the sides of the box -- the entire thing would look black.
Rendering 3-dimensionality is a skill that separates skilled artists from novices. The tip is to think about LIGHT. Light exists all around us, and it changes the ways that objects and colors look. When artists draw, they look at how light interacts with objects.
How to add 3-dimensionality
Your assignment is to draw a fun 3-dimensional illustration of an animal. You will use highlights and shadows to bring your illustration to the next level.
Digital Illustrator Tyson Murphy recreated this scene from the Sword in the Stone. Notice how the second image from the movie looks flat, but the first image has 3-dimensionality because of the HIGHLIGHTS & SHADOWS. Where is the source of light coming from?