Painting involves the two hemispheres of the brain. When we ponder on the balance sheet and the choice of colors, take a pencil in your hand and begin to shade the cell, then it immediately combines two processes: problem solving and fine motor skills. This kind of therapy, which trains the brain, accustoming it to work on simple tasks and gradually move to more complex tasks.
It is believed that the company The McLoughlin Brothers (en.wikipedia.org/wiki/McLoughlin_Brothers) invented the coloring books. They published a book called The Little Folks' Painting Book in collaboration with Kate Greenaway in 1880.