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.
Well, apparently they were back then and still are today for people of all ages. People are making the adult coloring books bestsellers on Amazon! At the time of this writing, eight of the top twenty books on the bestsellers list, are coloring books for adults. There must be something behind this recent growth in interest.