On Aug 26, the 22nd international book fair was held in Beijing, where a big coloring wall like the Secret Garden attracted visitors. [Photo by Mai Tian / For China Daily]

Yuan Xin, a psychologist at Tianjin-based Nankai University, explains that colors people use reflect their understanding of life, and coloring in can give rise to curiosity and a sense of feeling good. "The book gives people a short moment of tranquility and inner peace, and the 'white space' on a Secret Garden page represents the space in modern people's heart. They feel a strong sense of control and self-fulfillment while coloring in the space on paper," Yuan says.

Carl Jung, psychotherapist and founder of analytical psychology, was one of the first psychiatrists to say that coloring in can relieve mental stress. According to Jung, people can express their feelings of happiness or sorrow by choosing and using different colors.

In an interview with jiemian.com in August, Yang Baoyun, an editor with Hinabook, said: "Helping readers relieve their pressure is a selling point of Secret Garden abroad. Yet we have not emphasized that point while promoting the book in China. My understanding is that the book does make you feel better, because coloring in slows down your life."

It takes about 20 hours for careful readers to fill in all the colors of a single picture in the book. In the process, a reader can learn the importance of gradient ramp, color matching and light sensation.

"It's very difficult for people today to focus attention on one thing," says Sun Haoming, a painter in his 70s in Jinan, Shandong province. "The coloring book doesn't have the magic that the media ascribe to it. It's no different from shopping, eating and physical exercise.

As long as you focus on one thing, you will forget the unhappy things." It took him four days to fill in the colors of the patterns in one picture of the book.

Social media, too, have played a role in boosting the book's popularity. Secret Garden's lovers share photographs of their "works" on WeChat, the most popular social media network in China. Thanks to social media, the coloring book appears to satisfy some of people's needs, say experts.