Then you can also create a texture effect. Just cover the drawing with a light stroke first then follow with rough, think strokes to serve as details . texture. This can also be applied when you're using colored pencils. And you can also create a light, smooth look with crayons. Just color your drawing with very rough and thick strokes (this might make the drawing look messy at first) then rub off the excess color with cotton or scratch paper.
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.