The amateur artists can be found coloring in airport lounges, doctor's office waiting rooms and while they watch TV shows at home. They are forming coloring meet-up groups at libraries and coffee shops so they can chit-chat as they doodle. Coloring books for adults — a genre once considered little more than a novelty — are suddenly a big business, a bright spot in the financial results of publishers and retailers alike. Nielsen Bookscan estimates that some 12 million were sold in 2015, a dramatic jump from the 1 million sold the previous year.

Whether it is a short-lived fad remains to be seen. The new generation of books are typically filled with intricate black-and-white illustrations that are art themselves. While many find the act of coloring to be a calming distraction from hours spent tapping, swiping and staring at screens, some early adopters aren't exactly hooked. Several reviewers on Amazon.com found the need to stay in the lines to be anything but soothing. "Most of the pages are full of pictures that are so small I can hardly see the details to color them, which causes more stress than if I hadn't tried to color in the first place," wrote one reviewer of a popular coloring book on Amazon.

While adult coloring book sales have escalated in the United States in the past year, experts say the catalyst for the craze was the work of Scottish author Johanna Basford, whose 2013 title, "Secret Garden: An Inky Treasure Hunt and Colouring Book," began burning up bestseller lists with its detailed images of topiaries and flowers, and its "Where's Waldo"-esque challenge to find hidden items in the elaborate illustrations.