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.
Thanks to colorings the child perceives the world pattern, implements its own color discoveries. If a child paints the picture in unusual colors, you should not abuse him for that; it is his own opinion.