When I was in primary school, a nice teacher told me how to color something without it becoming scratchy. She told me to make gentle circular movements with a not too sharp pencil point (image).
Exercise 1: Try this without drawing outlines first. The shape that is created does not matter. Make sure you don’t get hard, sharp edges.
Exercise 2: Now try it out in a square, triangle and circle. Make sure you don’t bump into or overshoot the lines as in the bottom row.
You can also try coloring with the side of your pencil tip. In that case you need to change your pencil grip from Chapter 1. With this ‘flat’ grip, the back of the pencil rests in the hollow of the palm and the pencil lies much flatter above the paper. It’s a matter of trying it out.