Buying coloring for children, look for task-coloring, in which you need something painted in a specific color, something to finish, something to paste, draw a line through the maze, etc.
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.