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.
"The golden age" of coloring books is believed to be 1960-s, because almost all characters were depicted in them, as well as some products (like Planter Peanuts or Campbell Soup)