Pass the colored pencils, kiddoes: Grown-ups have muscled in on the fun.
2015 was the year of the adult coloring book.
At the start of the year, adult coloring books were a little-known niche item, but by December, bookstores around the country had cleared shelf space for the hundreds of different coloring books publishers were churning out.
There's no exact figure, but sales for adult coloring books were in the millions last year. The top ten bestselling titles sold at least 1.5 million copies, according to Publishers Weekly.
Coloring books for adults aren't new, but the market ballooned last year thanks to the intricate drawings of Scottish illustrator Johanna Basford. Basford published her first adult coloring book, "Secret Garden," in 2013, and it was a sneaky success, selling more than a million copies in two years.
When her follow-up, "Enchanted Forest," hit shelves in February 2015, the world took note. It sold almost a quarter of a million copies in its first month, and sold out within weeks.
"The floodgates opened," said Steve Mockus, executive editor for Chronicle Books.
Chronicle, in addition to publishing its own books, is the U.S. distributor of Basford's titles. "No one knew to what extent people wanted to color until they recognized the success of 'Secret Garden' and 'Enchanted Forest,'" Mockus said.
Press around the world marveled at the feat: Coloring books, for adults, flying off the shelves? How did that happen?
A coloring fan explains
Check the #coloring hashtag on Instagram if you don't believe coloring books are a full-blown phenomenon.
People all over the world share their finished pages — there are Harry Potter designs, traditional mandalas, floral landscapes, abstract shapes, even cats doing yoga.
Maybe your mom used to put your coloring pages on the fridge, but now the whole Internet is ready to appreciate how well you shaded those owl feathers.
The social aspect is part of what many new fans find appealing about coloring. Jenny Fenlason went so far as to start a coloring book club.
Every month, more than 30 people gather in a coffee shop in New Hope, Minn., to catch up and color. It's the Ladies Coloring Club.
"I thought coloring would be a fun way to get together and do something that engages a little bit of your creative side, but allows you to talk and hang out," Fenlason said. "I threw out the idea and a lot of people were interested."
She started a Facebook page for the club, and other chapters began to pop up. Now, people gather regularly in Minneapolis, Las Vegas, New York City and London to catch up and color.
Fenlason likes the simplicity and the low cost of coloring as a pastime. Her life is full: She has four young children. "It's relaxing to color inside the lines," she said. "And it's more attainable to me than knitting."
These are not your child's coloring books
Adult coloring books go far beyond cute animals and castles. (Though, if that's what you like, there's plenty to be found.)
The most popular coloring books on the market are works of art in their own right, with Basford's titles prime examples.
If intricate isn't your thing, there are licensed coloring books for your favorite franchises, from Harry Potter to "Game of Thrones."
Then there are the books that put the adult in adult coloring book. There are no fewer than five books on the market that let you color in your favorite swear words.
In the bookstores: Make room for coloring books
Powell's Books in Portland, Ore., got in on the trend early last year, but had a hard time keeping coloring books in stock.
Tom Luce, a new book buyer for adult art books at the store, said publishers were constantly having to reprint titles — no one had predicted the demand. Sales "went nuts" over the summer.
"It really just kind of came out nowhere," Luce said. "There have been a small number of specialized coloring books for years: Anatomy coloring books, botany coloring books. No more than half a dozen ... I started seeing the first of the coloring books in the catalogs, and I was like: 'What are these?' Where are they coming from? What am I supposed to do with these?'"
Powell's operates five locations in the Portland area, and "every store had to enlarge the [coloring] section from what it started with, and then enlarge it again and then enlarge it again."
Luce himself hasn't been able to resist.
"There are a large number of us who secretly enjoy a coloring book now and then," he said. "I do have a box of crayons at home."
The publishers' perspective: Why adult coloring books took off
Blue Star Coloring launched a year ago — just in time for coloring books' spring madness.
The idea to publish a coloring book started on a whim.
"Our CEO's wife is a doctor, and she came and told us: 'All the other doctors at the hospital are coloring the kids' pages because it's helping them blow off some stress as a break,'" said Gabe Coeli, Blue Star's chief creative officer.
Blue Star's first coloring book, "Stress Relieving Patterns," hit shelves in March. A month later, it had sold 15,000 copies. By May, it was a bestseller on Amazon. In total, Blue Star sold a million coloring books last year.
"It was insane. We were dumbstruck. We just kept sitting here: How can this be real? How can this be happening?" Coeli said.
Blue Star has now published 30 coloring books, ranging from the stress-relieving designs to "Yoga Kitties" and "Kaleidoscope Patterns." It has a dozen more in the works.
Coeli thinks the calming effect of coloring books is helping to fuel the trend. "I myself am a devotee of Zen meditation practice, so I immediately saw those parallels," Coeli said.
The psychology of coloring
Dr. Ben Michaelis, a clinical psychologist in New York City, wasn't surprised to see coloring books being hailed as stress relievers or self-care items. He'd been using them in his practice for years.
"[Carl] Jung used to prescribe coloring mandalas to his patients over 100 years ago," Michaelis said.
He realized the trend had entered the mainstream when his wife brought home Basford's "Secret Garden," ostensibly for their daughter. It was Michaelis and his wife, however, who ended up coloring in it.
"It's really the repetition that's key to the relaxation response," Michaelis said, explaining the book's appeal. He recommends coloring to people with anxiety.
"People that tend to be self-conscious are the folks that I think can benefit the most from this," he said. "Anyone who is older is also a good candidate, because it helps reduce the age-related losses in dexterity."
The decidedly low-tech phenomenon is a welcome activity for adults who spend their days staring at screens.
"This is part of a larger trend of people trying to unplug in one way or another," Michaelis said.
There is one group of people, however, who may not find peace in coloring. Coloring books like Basford's feature intensely intricate designs. People who obsess over details may get caught in the lines.
"People that are perfectionists may have a more difficult time with this, if they feel like it's stressful to 'do it right,' so to speak," Michaelis said. "That can actually end up being more stressful."
What's next for adult coloring books?
The coloring book phenomenon has been met with an ensuing wave of cultural criticism. Some critics have called the rise of adult coloring books a sign of an infantilized culture.
Susan Jacoby told The New Yorker that "the coloring book is an artifact of a broader cultural shift. And that cultural shift is a bad thing."
Sales numbers, however, seem to indicate that coloring fans don't care.
"Anything that's popular receives a backlash," said Mockus, the Chronicle Books editor. "You could argue that creativity is inherently childish — making things up and expressing yourself."
Mockus edits Chronicle's "Fantastic Cities" coloring book series, which features illustrations by Steven McDonald of cities around the world.
"The trick now is to try and find things that will come to the attention of coloring fans because accounts can't stock all the books that are out, let alone the ones that are coming," Mockus said. "They're being very discerning now in what they take."
Many publishers are already wondering what the next hit will be, if coloring books fade. Paint-by-number? Dot-to-dot? Stickers?
For now, coloring books are staying strong.
"Everybody's trying to figure out when the merry-go-round is going to stop," Mockus said. "But people are still discovering, and continuing to color and look for new books. We thought that maybe it would truly be a flash, and it hasn't been the case."