WASHINGTON (Reuters) - Coloring books, once seen only as a pastime for children, have become a growing craze for American adults looking to unwind or test their creativity with coloring pencils.
The trend has spawned coloring parties and clubs where grown-ups get together to color and socialize.
"You feel creative and you're like, 'Wow, I didn't know I could that, that looks pretty good,'" Washington resident Lydia Pesant said at a city library that hosts coloring parties every month.
With thousands of titles on the market, coloring books emerged as the most popular category of 2015, according to Publisher's Weekly, a trade magazine.
Nielsen Bookscan, a data provider, estimates that more than 2,000 coloring books were published in 2015, up from 300 in 2014.
Illustrator Teresa Roberts Logan said many people preferred the books to scrapbooking or knitting as a way to relieve stress because of the minimal investment required.
"You could have three or four color pencils with you or use your kid's crayons," said Roberts, who has produced her own book of paisley designs. "They're very accessible, they're very portable. I think they're user-friendly."
The adult coloring books are a far cry from the ones with simple pictures aimed at children. They often feature pages of complex black-and-white drawings frequently inspired by flowers, animals and geometrical patterns.
Scottish illustrator Johanna Basford's 2013 book, "Secret Garden: An Inky Treasure Hunt and Coloring Book," is often credited with accelerating the trend. It has sold 11 million copies worldwide.
Coloring books have made their way into online retailer Amazon's list of best-selling books, often occupying spots in the top 20.
The books can be found in special displays at bookstores and even in grocery stores, such as Whole Foods in the United States.
With Neilsen Bookscan estimating 12 million copies sold in the United States in 2015, the trend shows little sign of slowing. A new generation of coloring books taking inspiration from pop culture instead of more traditional designs is expected to hit stores in 2016.
"It's worth saying that whereas several years ago that would have been seen as something in just a younger market, today these are very much marketed and pitched towards an adult audience," said John Purves, deputy director of marketing and publicity at Politics and Prose bookstore in Washington.
Editing by Ian Simpson and Bill Trott