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.
Overlap Colors to Create a Blended Look. I like to lightly color the dominate color first and leave a bit of paper exposed for the wax of the second color to cling to. In the liver I colored (below), I started by coloring a layer of red, then I colored some brown on top. Then I colored more red over the top. I find this kind of layering to be really fun!