Consider this the coloring book version of applying a black and white or "antique" filter, as you would do on your Instagram feed. Working in pencil, marker or paint, choose a neutral palette. As you can see on the top image, shading with regular pencil on your coloring book image makes it look less like a coloring book image, and more like a hand drawing. By using a palette of browns, you can create an "antiqued" look on your coloring book pages, as if they are treasures from a different era.
Get out there and look at some of the fantastic finished coloring pages our members share with us each day. The internet is full of wonderful examples of coloring, drink it in, find new styles and techniques to try in your own pages. When you have finished a page, influence others or get feedback and tips to help you improve. Don't be afraid to put your work out there, the coloring community all share the same passion for coloring as you.