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.
When teaching a class on how to draw animals, we covered mostly graphite and colored pencils, but one of my students wanted to draw with crayons. It was fun cracking open the box of Crayolas and giving the class a demonstration. I shared the illustrations I had done for the book, and we analyzed the characteristics of drawing with crayons.