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!
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.