For best results, remove the coloring book page from the book before painting so the paint doesn't warp the pages below or seep through. Using removable tape, affix it to a rigid work surface to prevent buckling of the page as you paint.
Practice by shading two contrasting colors into one another. For example, take your red pencil and your green pencil. Starting with the red pencil, shade from left to right, dark to light. Next, take your green pencil and shade from right to left, dark to light. Work on blending them as seamlessly as possible where they meet in the middle.