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.
Let's first start with using crayons. It's usually one of the first coloring material introduced to little children because it's perfect for kids to use since there's jumbo sized ones that they can easily handle without breaking it in half. It's also a great material for bigger kids and even grown_ups because it has a wide range of colors to choose from.