Then you can also create a texture effect. Just cover the drawing with a light stroke first then follow with rough, think strokes to serve as details . texture. This can also be applied when you're using colored pencils. And you can also create a light, smooth look with crayons. Just color your drawing with very rough and thick strokes (this might make the drawing look messy at first) then rub off the excess color with cotton or scratch paper.
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!