A Variety of Pressure Creates Emphasis. If there are some areas you want to accent, you can press a little harder on the crayon and end up with a darker, more intense color. This is another good reason to color lightly for full coverage. For example, in this picture of the heart below, I pressed harder when tracing the lines within the heart and in the vein and artery openings. I did the same thing when coloring the muscles and the brain.
Build your colors gradually, and layer them. Apply a light undertone, and build darker colors slowly on top. Crayons can actually be more difficult to layer with due to the high wax content. Don't confuse them with colored pencils! If you want the "look" colored pencil, just use that instead.