'I never made money from the Green Lady,' says Tretchikoff's model

It was the face that launched a thousand drab sitting rooms into kitsch popular culture. Chinese Girl, with her otherworldly glow and decorative gold tunic, is one of the most reproduced fine art prints in the world.

The original painting, held in a private collection for half a century, goes up for auction at Bonhams in London next month, when it is expected to fetch up to £500,000. No one is more intrigued about its destiny than the original "Chinese girl" herself, who was a 17-year-old working at her uncle's launderette when she caught artist Vladimir Tretchikoff's eye.

Today Monika Pon, a South African whose parents emigrated from China, lives in Johannesburg and has come to terms with her face being as ubiquitous as that of the Mona Lisa – even though it brought her neither fame nor fortune.

"I've got no money from the painting," said Pon, who is now about 80, though declines to give her exact age. "I was so stupid, so young. What did I know about business? He said, 'I'm Tretchikoff', and I'd heard of him, I knew who he was. He said, 'I would like to paint you.' I was nervous, but the look in his eyes told me I couldn't say no."

The encounter came in Sea Point, Cape Town, in the early 1950s, when the Russian-born Tretchikoff was regularly sketching ballet students and looking for other models. A ballet teacher put him in touch with Pon, a renowned local beauty. She recalled: "I don't think Tretchikoff was so famous in those days. When I met him, I got on with him very well. He was a nice person, straightforward. My uncle had a laundry in Cape Town and once or twice a week Tretchikoff came to visit."

The artist paid her about £6 for six weeks of sittings before an audience of his art students. When the Chinese Girl painting was unveiled, Pon was less than impressed. "I'm not green! I didn't understand what it meant. I told him and he said, 'What don't you like?' I said, 'I don't like the green, it makes me look ill.' He laughed."

Even now she objects to the painting's popular title of "Green Lady". "There is no such thing as the Green Lady. They made a mistake. There's going to be an auction and I think the name should be right. People say the Green Lady or the Blue Lady, but they should get it right."

Whatever it's called, the painting became a worldwide phenomenon and postwar classic, adorning innumerable living room walls in Britain alone. Tretchikoff claimed that by the end of his career he had sold half a million large-format reproductions. Pon's face can be found on mugs, wallpaper and other collectables.

Yet the success had no impact on her life; her identity as the "real" Chinese Girl was only revealed in 2011. She worked in the shipping industry and in a grocery shop and fish-and-chip shop. She married and moved to Johannesburg, but divorced young and met a new partner. "I've had the same boyfriend for 40 years," said the mother of five. "I said I didn't want to get married again." She lost touch with Tretchikoff until the late 1990s, and never posed for another painting, but retained artistic interests of her own. "I like to draw and I can paint pictures of my children. I like the old artists: Leonardo da Vinci, Michelangelo. I don't know the modern artists, except Tretchikoff."

Celebrity has come late in life, but she has learned to deal with it. "I went to a hospital and everybody there knew me; I didn't know anybody. I heard daughters telling their mothers that the painting is the biggest seller in the whole world."

The original painting was bought directly from Tretchikoff by an American woman in Chicago, when he was touring the US in the 1950s. It has remained with her family ever since, but will be sold by her granddaughter at Bonhams on 20 March for an estimated figure of between £300,000 and £500,000.

Prior to that, it will be displayed for four days in New York and for three days in Johannesburg, but Pon – who was reunited with it at a Tretchikoff exhibition at Cape Town's National Gallery in 2011 – has recently been ill and bed-bound, so may miss her chance to glimpse it again. "I don't mind where the painting goes, although I'd prefer it to be a museum; I'd like it to be on display," she said. "South Africa would be nice. I've got three daughters and one of them said that, if she had the money, she would buy it if she could.

"I'm very anxious to see how much it will go for. I think it will be a lot. Good luck to the person who gets it if they've got that much."

Chinese Girl was generally despised by the critical establishment and Tretchikoff, who referred to his home in an upmarket Cape Town suburb as "the house the Chinese Girl built", was puzzled by its massive popularity. "I still cannot explain the mystery of my painting," he wrote in his autobiography, Pigeon's Luck. "I would have never believed anyone who told me … that one day I would paint a picture that would appeal literally internationally – not only to the European races, but to Orientals and Africans as well." Local researcher Boris Gorelik, the author of a forthcoming book Incredible Tretchikoff, said: "I think the Chinese Girl has an impenetrability that mesmerises you. When you look at the Chinese Girl, she reflects your thoughts back at you, like the tiger in Life of Pi. Some think she's an eastern seductress, the dragon lady; others believe she's a humble oriental beauty who's resigned herself to her fate.

"Then there's a million of questions to ponder over as you stare at her from the sofa. Why is she green? Why are only the face and the hair fully painted? Why does she wear a 19th century Chinese gown and Max Factor lipstick? How old is she? This picture can keep you busy for years and you still won't find the answer. It's challenging but in a pleasant, comforting way." He added: "What's also important is that this image goes so well with the retro chic interiors people love so much these days."

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