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.

Coloring pencils or markers are useful for the development of fine motor skills. After painting the small objects, the child learns not to go beyond the boundaries of the drawing and trace the thin lines.

Military Color Pages