In 1940 some movie stars became so popular among the coloring books industry. Among them Judy Garland, Betty Grable and Margaret O'Brien.
There is another advantage of colorings for adults. Most of them are very nice and include impressive black-and-white contours that can be turned into a work of art. Imagine the feeling of satisfaction that you get when you have finished drawing and will be able to appreciate the beauty of the masterpiece created by you personally.