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.
When I was a child I just loved to draw and paint but, as I remember, before I even could begin to draw accurately I used to color in. The next stage was to learn to draw using stencils and then I graduated to be able to draw freehand which provided me with an absolutely fantastic sense of achievement. I went on to love drawing but the point I am trying to make is that it all started with the simple yet effective art of coloring in!