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.
The themes of adult coloring books are usually intricate and centered more around adult images, not your childhood counterparts that included bunnies, super heroes, and farm animals. You would expect to see geometric patterns, psychedelic patterns reminiscent of doodle art type pages, fantasy images including angels, dragons, goddesses and mermaids, and ancient designs of religious and spiritual nature.