Here are some tips on using crayons for effects in coloring: You can start with a light layer of the color you want to use first. Then you can cover that with darker or lighter color to add life to your drawing, just like the wings on this butterfly.
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.