The scene backstage at the Sass & Bide show at New York Fashion Week is one of glamorous chaos. There are models scrambling into clothes, production staff testing sound and lighting to dramatic, somewhat cacophonous, effect and even an MTV crew filming an episode of a teen drama. Amid this frenzied activity, a petite woman in jeans with hair the colour of black coffee, who looks like a quietly glamorous "mom", is providing the calm centre of the fashion storm. Bobbi Brown, renowned make-up artist to everyone from Sienna Miller to Susan Sarandon and founder of a successful, eponymous cosmetics brand, radiates a most un-fashiony serenity as she demonstrates the maquillage she has devised for the show on a model. She dabs at her face almost like a Renaissance painter, palette in hand, before her "élite team" of make-up artists from around the world set to work on recreating the effect.

The show is called Rainbows For Kate, and is a tribute to a late friend of the design duo behind Sass & Bide. It is an uplifting celebration of life, colour and harlequin details, and the make-up to match is a palette of mauves and shimmery silvers, with the faintest, ultra-feminine pink blusher. The idea is that the colours "let the dresses dance", explains Brown, and the look has a lot more disco sparkle than the natural "no make-up" make-up look with which she is synonymous.

The atmosphere is crackling with concentration, but the kind of harassed, bossy air that pervades the behind-the-scenes areas of many shows is notably absent. In fact, Brown's team seems implausibly relaxed. But then Brown herself seems almost too calm and friendly to be true. She describes herself as being "a normal person. A lot of people in the beauty industry are like, 'Darling it's fabulous, it's wonderful.' That's not real. People are happy to hear the straight talk." Many celebrities – or at least people in the public eye – refer to themselves as being down to earth and approachable, when in fact it is instantly apparent that they are anything but. However, Brown seems genuinely warm and upbeat without being saccharine – qualities that enable both celebrity clients and female customers of her products to relate to her, and that allow her to switch from blending glitter on a model's eyelid to chatting about anything from journalists' make-up techniques (her verdict: "they don't wear much, perhaps they think they are too cool for it," before adding, politely yet convincingly, "but yours is very good") to her famous clients present and past. She's done Carla Bruni's make-up "many times. She's very nice. The first time I met her she said, 'My boyfriend just bought me a car.' I replied, 'Oh that's nice, what does he do? He plays guitar? Is he famous?' And she said, 'Yes, he's Eric Clapton.' She's very funny ... I mean, where can you go after Mick Jagger ... the president of France, hey?" On the subject of another former First Lady, Brown is less enthusiastic. What does she think of Hillary Clinton's make-up? "I don't comment on political beauty," she says, adding with a wry smile, "but I think Michelle Obama's make-up is perfect."

A young fan who has found her way backstage holds forth on her admiration for Bobbi Brown. "I just think she's amazing," she gushes. "She really inspired me when I was growing up. She advises girls to be healthy and she's got really strong morals." Brown is full of wholesome advice – such as always be on time, drink lots of water and don't sunbathe – but these seem more like sensible adages than morals. However, such is the quasi-religious fervour that beauty and skincare increasingly excite – witness the number of people described as skincare gurus, the plethora of spa brochures that invoke Nirvana – that perhaps it's not surprising that the girl interprets Brown's vision as a beauty belief system. Even women who take a more pragmatic view of cosmetics can get quietly enthusiastic about Brown's refreshingly positive view of ageing, foundations that suit pale to dark skin tones, and a range of natural colours that make it pretty darn hard to get one's maquillage horribly wrong.

Brown's flair for cosmetics began with a degree in theatrical make-up from Emerson College in Boston. Frustrated by the garish make-up that was popular at that point in the 1980s, she decided to develop her own lipstick shades after a chance meeting with a chemist at a photoshoot. She started with one lipstick and nine other brown-based lipstick shades followed soon after. When they went on sale in the luxury New York department store Bergdorf Goodman in 1991, she expected to sell 100 lipsticks in the first month and ended up selling 100 in the first day. Other beauty products followed – she became famous for her natural-looking, yellow-toned foundations, and she sold her business to Estée Lauder in 1995 for an undisclosed sum, while retaining control of the company. In 1994 she introduced a skincare line and has written several bestselling books on beauty – the most recent being Living Beauty aimed at women in their forties, fifties and sixties.

Brown says of the way that her success has spiralled – her business has doubled in the UK in the past three years – that, "If anyone had asked me whether I had a dream then I would have said, 'No', because I would have wanted to run away. It's much bigger than I ever wanted it to be." Does she ever feel overwhelmed by the pressure of running her business? "I don't feel pressured, I work really hard at balance. And I have an amazing husband who always helps me. On an average week I will be in the New York office on two days, in my home town of Montclair, New Jersey, at the new store two days, and one day I work from home and do the kids' school things."

Brown has so far resisted bringing out a "super cream" – a moisturiser with a grand price tag matched only by its extraordinary claims to reduce wrinkles. She says, "Our creams are expensive enough – they're about $50 or $60 – but some of the big stores I'm stocked in have said, 'Please come out with a cream that's around $300.' They like them because there is a customer for it, and it brings excitement into the store." Brown says she wouldn't feel comfortable launching one because, "there is no miracle broth out there. I do think my creams are worth it, but they are not going to get rid of a line or a wrinkle – nothing is. They're going to hydrate and smooth."

Brown avoids endorsing or condemning plastic surgery, but tells me that, "most of the women I've seen that have had it done look weird – like a Picasso painting." She has had botox a few times, and "a whole bunch of laser things", and admits it's easy for her to speak out against full surgery because "I look young for my age so it's hard to say how I will feel in 10 years' time." At 51, she has a fresh, natural complexion that contrasts with the tendency of so many American women to favour an impasto application of tan foundation.

She wasn't always this comfortable with her appearance – as a young girl she idolised the blonde-haired blue-eyed Cheryl Tiegs – but as a 30-year-old make-up artist surrounded by Cindy, Naomi and Helena at a swimsuit show she looked at their perfect figures and said to herself, "OK Bobbi, don't go there; you can't feel bad about yourself just because you don't look like them. Actually," she continues, "those models don't exist anymore ... most of the models today are very thin and almost odd-looking. When my three boys or my husband meet models, most of the time they think, 'Euw!' I walked out of a casting yesterday and the models were all basically pencils with heads on them. They are getting thinner and it really does bother me." Brown then tells me about a model who confided in her about her eating disorder, who she plans to ask if she wants to be a make-up artist. "I think there are a lot of girls who have become [make-up] artists who have found it has really healed their self esteem because it's something creative that you control."

It is just such a blend of creativity and control that has made Brown into everyone's favourite multi-million-dollar make-up artist next door.

My three golden rules,by Bobbi Brown

Be natural: The secret to natural-looking bronzer is only applying it to the parts of your face that the sun would naturally hit – cheeks, nose and chin. To ensure a seamless look, make sure that you apply the bronzer with a brush that has a big, fluffy head. The most natural-looking bronzers have predominantly brown tones to them. Avoid bronzers that are orange-toned or frosted. Most companies offer light, medium and dark shades. Pick your shade based on how you tan naturally. If you've been blessed with a porcelain complexion, use a soft pink or apricot blush to warm up your skin.

Less is more: The lazy days of summer are the perfect time to pare down your make-up routine and let your skin breathe. However, heat and humidity can trigger breakouts and make-up meltdowns. Make sure all products you use are as sheer as possible; there's no need to cake heavy foundations or powders on to the skin. To combat heat and humidity switch to an oil-free foundation or an oil-free tinted moisturizer and a gel cleanser.

Colour vision: Transparent, shimmery colours are a great choice because they leave the skin looking soft and breezy. Tans and light blues are reminiscent of sand and the ocean, and they make a great eye-shadow combination for anyone who craves a little bit of colour without being overwhelmed. Put away your matte, full coverage lipsticks and try a tinted lip balm. It's a quick, easy and modern way to add colour to your lips while protecting them.

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