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.
I try not to color for more than 20 minutes without shaking the stiffness out of my hands! Look after your hands and wrists! Before during and after you color, make sure you stretch your hands and wrists. Keeping your fingers and wrists locked in the coloring position for too long can lead to stiffness in the joints, aches and pain.