The Devil Wears Prada, by Lauren Weisberger, was published in 2003, and the film version was released three years later, yet one line continues to resonate. Or rather, one concise theory involving a blue jumper that claims to illustrate the structural and economic connection between high fashion and the everyday, referred to in popular culture as the “cerulean blue” theory.

If you have seen the film you will remember the speech in which icy fashion magazine editor Miranda Priestley (Meryl Streep) articulates to her cynical assistant, Andy (Anne Hathaway), precisely why the younger woman is wearing a blue jumper: “In 2002, Oscar de la Renta did a collection of cerulean gowns, and then I think it was Yves Saint Laurent – wasn’t it? – who showed cerulean military jackets ... so you’re wearing a sweater that was selected for you by the people in this room.”

The trickle-down effect has been a fashion truism for decades, celebrated by the song Think Pink in the whimsical 1957 fashion film Funny Face. Here pink is couched as something more arbitrary, an edict from a magazine editor to the women of the US to banish their red and blue clothes from their wardrobes. The editorial team celebrate by painting the office doors a shade of rose.

Still it is notable just how often the theory is cited now, often in defence of the fashion industry. Last week, influencer and broadcaster Derek Blasberg tweeted to his 276,000 followers: “How many times have you used the ‘cerulean blue belt’ explanation from The Devil Wears Prada to validate the importance of the fashion industry?”, while in June, New York Times fashion critic Vanessa Friedman referenced it in a searing defence of why she reports on couture.

It is curious then that this cerulean blue treatise – so referenced by the fashion industry – was in fact fabricated by scriptwriters. Oscar de la Renta’s autumn/winter 2002 collection actually showed smart sportswear in grey, taupe and black, while Yves Saint Laurent’s collection was almost entirely black. The screenwriter, Aline Brosh McKenna, was a comic writer who went on to co-create Netflix’s Crazy Ex-Girlfriend, which rather throws the validity of the whole idea into question.

Still, it does sometimes hold water. As a fashion journalist I can vouch for its gist: that regardless of how immune you think you are to fashion, if you buy clothes, you are indebted to someone else’s choice. Arguing that you are oblivious to trends is a fashion choice in itself.

Take yellow, one of the biggest trends of 2018. It arguably exploded on the red carpet in 2015 when Rihanna wore a yellow dress by Guo Pei to the Met Ball. The dress became a meme, and a genesis moment for the shade, which was revisited by Alessandro Michele for his Gucci cruise 2016 collection, appeared again (albeit in a neon shade) in Balenciaga’s pre-fall collection (the first under Demna Gvasalia) and finally earned the approval of Beyoncé, who wore a yellow 2016 Roberto Cavalli in the Hold Up video.

Since then, the colour has worked its way onto the lucrative backs of celebrities at formal occasions – see Amal Clooney at the Royal wedding, and Kate Middleton at Wimbledon this year – and eventually, on to the high street where it is now a bestseller at Topshop, Whistles and M&S.

Rihanna arrives at the 2015 Met Ball Photograph: Timothy A. Clary/AFP/Getty Images

“It has taken a long time to take seed,” says Tony Glenville, fashion author and forecaster. “But that’s probably because of the association.” Compared with another very popular colour, millennial pink, yellow has a rep for being tricky to pull off. “All we needed was a new name,” he says, which is probably why it’s now called either as buttercup yellow (by trend forecaster WSGN) or Gen Z yellow (coined by the Man Repeller blog).

“There is good historical evidence showing that industry and consumers tend to prefer colours that reflect the tenor of the times,” agrees design historian Regina Blaszczyk, author of The Colour Revolution. “The most famous example is that of American automobile colours in the 1920s and 1930s. After the stock market crash there was a dramatic switch to darker hues. In many respects, cars followed the hemline theory: up and bright in good times, down and dark during an economic low.”

It’s one thing to reflect the times, but another thing entirely to suggest that only designers can decree the colours (and trends) that will succeed. Yellow’s rise, for example, has been helped by other factors. Glenville cites the heatwave; it’s not that we need cheering up as much as we need to map the climate.

“The uptrend in soft buttercup yellow is also an evolution of 2017’s mass adoption of pink,” says Hannah Craggs, a forecaster at WGSN, suggesting we latch on to colour as a way of making sense of the world.

Glenville thinks another factor in the rise of yellow is its association with nature: “It is an eco colour, along with blue and green,” he says. “Sustainability is very big now, and I think on some subliminal level, an awareness of that has seeped onto the high street – but it still had a catwalk story to support it”.

As it happens, cerulean blue was likely chosen for the film because it looks good on screen, and it sounds legit – and because six years prior, cerulean blue was named colour of the year by the Pantone institute. Each year, the colour-forecasting company attempts to capture the chromatic mood, which it describes as “a snapshot of what we see… in our global culture that serves as an expression of a mood”.

Amal and George Clooney at the wedding of Prince Harry to Meghan Markle. Photograph: WPA Pool/Getty Images

Announcing the colours several years in advance, Pantone – and other forecasting agencies such as WGSN – actually predate designers’ collections by at least a season. So rather than designers dictating, on a whim, the colours they “are feeling” (a phrase that crops up with startling regularity) they may well have been advised by an agency ahead of time.

That is a much less romantic image of what the industry does. So, too, is the use of algorithms. There are fashion search engines that track just how often a colour (or trend) is searched for, and feed this back to the designers, who then create or order more of the same.

It is also about commerciality. Indeed, if a colour is in demand, a dye house will come up with more tones of the same colour so it becomes a self-fulfilling prophecy. But a shop that only sells shades of the same colour doesn’t work. Glenville cites Parisian designer Claude Montana who tried to sell five shades of red (he went bankrupt in 1997).

“All of the evidence suggests that colour forecasters don’t necessarily look for the one colour that will take off, but focus their energies on directions – the directions of the cultural mood, which in turn are reflected in the palette,” says Blaszczyk. “Pantone is merely suggesting that a certain colour seems to embody the zeitgeist.”

Of course that includes not just the catwalk but popular culture and politics, too. Orange became so popular when Orange is the New Black first aired in 2014 that a Michigan jail changed the shade of its uniforms for fear that prisoners might be mistaken for trend-followers. Shades of blue are used to depict clarity in the Facebook and Google logos; Republican red, of course, has shifted from a powerful colour to one of fear in the wake of Trump’s ascent.

Colour forecasting began in 19th-century France, later becoming a business-to-business service, says Blaszczyk, although it really took off in the early 20th century in the US. One of the earliest forecasters, Margaret Hayden Rorke, noting a fad for First Lady Pink (a sort of millennial pink) based on the colour of Mamie Eisenhower’s 1953 inaugural gown. Given the timing, this may have influenced Funny Face (the two are four years apart).

While people do gravitate towards colours for subjective reasons, and some don’t even notice what colour they are wearing, it has become increasingly hard to distance yourself. In Priestley’s – and McKenna’s – defence, cerulean blue did crop up in Nina Ricci’s and Ralph Lauren’s 2006 shows. No trend is an island – and it seems no colour, either.