These extra sheet of paper will stop your coloring being affected by the texture of the surface you are working on. Colored pencils will easily pick up even the smallest dents, divots or wood grain textures. For smoother coloring give your page that little extra cushion! It is also useful to add paper between your page and the next when you are using a coloring book, the paper protects the design underneath from being damaged.
Consistent Pressure Gives a Uniform Look. Keeping a consistent pressure with the crayon on the paper gives a uniform, pleasing look. If you're pressing hard, your hands will get tired quickly, so I prefer to press lightly. Interestingly, being aware of how much pressure you're applying seems to be really hard for younger children. It must be tied to motor skills in some way. It won't hurt anything to tell them this tip, but if they aren't implementing it, just let it go.