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.
One day I suggested that my friend and her kids and I should have a coloring party. We could have hot dogs, chips and ice cream and all color together, with the TV off, of course. We had a blast and we plan to do this once a month. Since then I have made it a practice to color in my books at least once or twice a week. This time is very relaxing for me and gives my mind a much needed break and its just plain fun. I think it is sad that we abandon, as adults, the fun things that we engaged in as kids. Just because we grow up doesnt mean we have to stop having innocent fun. I know it sounds simple, but try picking up a coloring book with images in it you can relate to and color inside or outside the lines if you want. Maybe even just scribble, but allow yourself to get in touch with the less serious side of yourself for a little while.