Today there is a large set of coloring books. You can find coloring-cursive books, books outliners, for boys (all sorts of appliances, machines, robots, etc.), for girls (fairy-tale characters, dolls, funny funny little animals). There's even a wallpaper coloring books that opens a wide field to have enough space for young artist.
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.