At Bandier, a Manhattan fitness boutique, one of the most popular racks is packed with leggings that range from pretty to pretty funny: They are covered in loud prints of cherry blossoms or green kale, neon-pixel designs or emojis.

The era of simple, black leggings or yoga pants as the universal workout bottoms appears to be over. Instead, yoga classes, boutique fitness studios and running trails are filled with Spandex leggings in look-at-me prints, often from brands that aren’t exactly household names. Devotees swear the patterned leggings are more flattering—and just more fun—than their drab brethren.

“We can barely keep them in stock,” says Donna Burke, managing partner of Atlanta Activewear, in Georgia, which recently sold out of $74 “Malibu” scene-printed leggings from Onzie, a four-year-old Los Angeles company. In the past year or so, sales of printed leggings “went from very minimal to an absolute have-to-have trend,” Ms. Burke says.

Liz Kelley, who works at an Austin, Texas, software company, wears leggings that have one-half of a tiger’s face on each thigh. “They were so ridiculous, I couldn’t not buy them,” Ms. Kelley says. “They make people laugh.”

Not everyone wants to highlight their thighs with an explosion of color. “I hate patterned leggings” and “patterned leggings are ugly” posts abound on social media. “I’m just the type of person that likes black or gray or navy,” says Melissa Scott, a payroll director in Augusta, Ga., who practices yoga and lifts weights. Patterned leggings, she says, “draw too much attention when you’re in the gym. They’re just too loud.”

Yet many people who wear them started out as skeptics, brand and store owners say. Rebecca McCrensky, founder of Altar Ego, an Andover, Mass., athletic-wear company, says women might start out buying a “gateway” legging in a black-based print, such as her design with a red-and-white skull on the side.

Many printed leggings “have horrible hanger appeal,” Bandier owner Jennifer Bandier acknowledges. “But I always say to people, ‘Those leggings don’t look like they’re anything, and then you put them on and you’re like, Oh my God, I look like a supermodel,’ ” she says.

The busier the print, the more flattering it is, because the eye follows its movement, proponents say. “Prints hide everything and enhance all the right parts,” says Onzie owner Kimberly Swarth. She says a high-quality fabric keeps a print from looking stretched on larger parts of the leg. Ms. Burke says Onzie’s Malibu design looks great on because the pattern’s darkest part is high on the leg, with lighter flowers and sky lower down.

What to wear with patterned leggings can vary. Some women wear a simple fitted tank top or loose sleeveless T-shirt. Some wear only a matching sports bra. For more coverage, tie a sweatshirt around your waist, Onzie’s Ms. Swarth says.

Sales of women’s leggings advanced 18% in 2014 to $1.1 billion, with sales of “active” leggings growing twice as fast as leggings overall, according to NPD Group. In the crowded “ath-leisure” segment, prints do the trick of offering shoppers more reasons to buy.

When women embrace the trend, they often buy multiple pairs, says Atlanta Activewear’s Ms. Burke; people notice when you wear the same vibrant print several times in a row. “You can only wear them once every two or three weeks because they stand out,” agrees Kelly Hershman, a 37-year-old spin and barre instructor in Dallas who calls leggings a “passion” and says she owns more than 80 pairs.

Liz Holt, 47, says she didn’t own any patterned leggings six months ago, but was converted after seeing others wear them. “I had the mentality before that pattern would make you look bigger, but they really don’t,” she says. She says she feels confident in front of the mirror during a workout. One print—of audio speakers—didn’t work for her, but she found six patterns that did, including a design of lightning over Paris.

Many aficionados discover small leggings brands like Emily Hsu Designs, of New York, on Instagram. The patterns on her leggings include “Bang Bang Graffiti,” a cartoon-like print including nail polish, kittens and the words “Bang” and “Zap” in a rainbow of colors. Ms. Hsu started her company less than a year ago as a one-woman operation, after friends kept asking about leggings she was making for herself and her daughter.

In her first month of business, July 2014, Ms. Hsu started posting on Instagram and sold 50 pairs; by January she was selling 400 a month, for around $50 each. She says she expects 2015 gross sales to exceed $250,000; her leggings retail for around $50. Customers come from all over the U.S., England and Thailand.

“It happens to be that right time, where everyone is really embracing the printed legging thing,” Ms. Hsu says. Many people wear them outside the gym with a cute top and ankle boots, she adds.

“You wouldn’t think I could wear donuts on my leg,” Ms. Hsu adds. But she did wear donut-printed leggings to yoga recently. “People loved them,” she says.

Zara Terez Tisch, founder and chief executive of activewear brand Zara Terez, says some of her company’s original prints began as photographs taken with an iPhone. There are patterns of multicolored cassette tapes and lavender hydrangea, and the company recently collaborated with Toast, a rescue spaniel and social media sensation, on a collage of images of the dog, tongue out and wearing various outfits.

Anna Kaiser, owner of Manhattan dance-based fitness studio AKT in Motion, has sold patterned leggings at her studio since she opened in 2013, mostly from smaller companies like Zara Terez and Cândida Mariá. “There was so much space in the fashion fitness realm for them to emerge,” Ms. Kaiser says.