Sales of books in US and UK grew in 2015, and part of that growth has been attributed to sales of adult colouring books. https://goo.gl/WjRVba
It has been said that these books help people de-stress and find calm. One of these books has sold over one million copies. But why not just paint pictures or sketch something? I suppose some people have difficulty drawing even a stick man so colouring between lines drawn by a professional is the next best thing. Is this a condemnation of modern education? I know many people who cannot draw even a map to save their own life. These basic skills are not being taught at schools as governments believe that humanities is a waste of money, and what children need is more science and engineering to meet demands by the workplace.
I volunteer at a psychiatric hospital one day a week. Most of the patients there are sane, well-adjusted people who suddenly blew up at work or at home. What do the doctors have the patients do? Colouring books of pretty pictures. Go figure!