This skill will develop gradually as they go from struggling to stay inside the lines, to perfecting this fine motor activity. You won't be hearing that they are bored. Lots of children at higher classes cannot handle the pressure of studies because they don't know the exact process or method of learning because their preschool learning had been very much mechanical and dull.
If you have mostly toddlers, pre-schoolers, or kindergartners, you will want to stick with basic, simple coloring pages that have large expanses to color and simple things like pumpkins or witches or black cats. This matter will be depicted on the page thus developing the child's coloring skills; the fine motor and coloring skills will thus be developed because these skills are highly important especially when dealing with the preschoolers. It is that simple.