Always Color in a Consistent Direction. This is my first tip. It will make a bigger impact on the end result than anything else and any age can use it! The changes in direction, shown on the red kidney below, on the left side, are what make it look sloppy, even though I carefully stayed in the lines. Coloring in a consistent direction, as seen on the other kidney, is easier on the eyes and ends up looking very tidy!
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!