Bobbi Brown tilted her tanned face as she sized up the girl seated before her. The model was a long-necked woman with freckles and wide eyes that had been rimmed in navy. “You know what?” Ms. Brown asked at last, prompting the smattering of underlings hovering by her side to lean in closer. “She’s wearing too much foundation. And you know why?” Ms. Brown grinned. “She’s 14! She doesn’t need it. All this foundation makes her look dead.”

After dousing a sponge triangle in makeup remover and running it all over the girl’s face, she followed up with purple shimmery eye shadow, then smeared concealer underneath her eyes. The girl’s skin was coffee-colored and the concealer a greenish yellow, but no matter. It blended in perfectly.

As the CEO and founder of Bobbi Brown Professional Cosmetics Incorporated, a multi-million dollar cosmetics firm, Ms. Brown, 47, didn’t need to work backstage during Olympus Fashion Week to pay the rent. But she signed to create the look for four shows because it’s a kick. “I do it because I love it, and it gets me excited,” she said.

Though in the past she was paid for each show, she now often volunteers her services to up-and-coming designers. It’s a trade-off for turning the shows into a classroom; a team of Bobbi Brown makeup artists comes along for the ride. “It’s a learning process for my artists,” she said. “I have my best ones here to see how it’s done.”

At the beginning of a show, her team gathers around to watch her work on one model, and then they all split off to try their hand on other models. Ms. Brown sees each model for final approval – and often removes the makeup and reapplies it herself. The makeup from the Bobbi Brown line is known for being pared down and simple, and Ms. Brown dresses in keeping with that. In jeans, a black pullover top, and flat sandals, she was the most approachable-looking person backstage at Sandoval. At just 5 feet tall, she stood as a beacon of calm in the pre-show chaos.

Apart from the test smudges of eye shadow on the back of her hand, the only makeup she had on was a little blush over her bronzed skin. “I keep trying to wear more,” she said. “I was on the beach with my sister-in-law, and she was wearing my liquid liner. It looked great, and I thought, ‘I should wear that.’ “

Usually at fashion shows, makeup artists aren’t shy about adorning faces with applique tulips or 6-inch-long eyelashes, but Ms. Brown keeps things basic and wearable. She consults with the fashion designers before the shows, and comes up with distinct looks for each collection. At Sandoval, which featured simply cut black-and-white togs, she gave all the models salmon lips and navy-lined eyes, with the line at the bottom thicker than the top. “He wanted foundation on the brows so it looked like they didn’t have eyebrows, but I talked him out of it,” Ms. Brown said. “He also said no blush, so I pinched their cheeks before they stepped out.”

Ms. Brown came up with two looks for Charles Nolan’s collection – one for the teenaged models, the other for the few older women featured in the show. The young girls were touched up with glittery lips, pale eye shadow, and a generous application of black mascara. A variation for the older models involved a lined eye. To create an outdoorsy look in Roland Mouret’s show, she used cream and powder blush on their cheeks with lipstick to match the models’ own natural lip colors. And at Lela Rose, Ms. Brown focused on pretty and pink, which happens to be the theme for the new spring collection her company is unrolling in March.

Ms. Brown cautioned against interpreting the fashion show looks literally. “You have to realize the models are under 20 and you still have to do what’s right for you,” she said. “If the models don’t have eyeliner on, that doesn’t mean you shouldn’t have liner on.”

She suggested taking one bold element from runway makeup and incorporating it into an everyday routine. Her suggestion for this fall is matte, clay-toned lipstick and pink, glittery eyes and lips for spring.

And there’s some good news, too. The cosmetics guru insisted that makeup can look better on a civilian than on a 14-year-old fashion model. Young girls often have pimples that are hard to cover up, and they can’t get away with everything: “Sometimes they look like they dipped into their mom’s makeup.”