You have a greater tool - your PC - that can be helpful to get the printouts of diverse colorful sheets. Guide the children, exclusively during the holidays to add illustrations to the colorful-page, and showcase them around the prominent places of the house. This will give them, a sense of pride and joy for their creativity, and would lead to further productive pursuits. Thus, you will witness that there are massive advantages of coloring-page relatively to coloring books. Coloring sheet are economical, costing you only a couple of cents in ink and paper rather than a few dollars for a coloring book.
Adult coloring books are all the buzz of late. It is clear that it revolves around colored pencils or crayons and coloring pages. But, it is just coloring, right? How can something like staying within the lines be a benefit to me? I grew up during the time of doodle art - remember those? You got a pack of felt markers and elaborate black line drawings on a number of themes. I would spend hours coloring these in! Little did I know then that coloring pages were a benefit to my well-being.