Some colors are more transparent than others. Some colors are very opaque. Test your colors on a separate piece of paper to see how they work together. Too many layers of crayon can make the colors resist each other. This can make it harder to add more color. Use as few colors as possible to get your end result. Scratch out small lines and details (such as hair, the veins in leaves and flowers, etc.) with a craft knife. Crayon is excellent for scratching due to the wax. Do not get frustrated! You will need practice to get the look you want. It's worth it!
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!