Coloring helps the child to learn how to control the scale of his hand. Please note that when the baby is just started to draw he does not monitor the boundaries of the sheet, and the picture is not painted on the outlines, and even beyond.
Painting will be very helpful for people with post-traumatic syndrome, or those who are constantly in a stress state. The fact that such activity calms the amygdale, i.e. the area of the brain that is responsible for generating emotions. When you focus on coloring pictures, you give your brain a chance to rest and relax.