Try to use a themed coloring. For example, a series of coloring books about animals, buildings or machines give children an idea of the objects of the world. Coloring of fairy tales or cartoon characters, are good because they allow the child to "talk" with their favorite characters and express their attitude towards them through colors.
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.