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.
If while coloring your child goes out of the outline, it will not be so noticeable. When the child sees how cool his result is, it will inspire confidence in him and will be a good stimulus for further work. Good coloring are those with the text in it. The text introduces the kid with the game, the kid will be interesting, while the text will expand its horizons.