One of the hardest colors to make is not just blue—it's actually cerulean
The Devil Wears Prada made a cerulean sweater a moment in fashion history. But its scientific history is much older, and more fascinating.
Some people might see a deep sky-blue sweater and think that it is simply blue. But there are many kinds of blue, from navy to baby. And in the case of one specific sweater in The Devil Wears Prada, “it's actually cerulean.” In the famous scene, Meryl Streep’s character, Runway editor Miranda Priestly, proceeds to give the hapless, cerulean-clad assistant Andy an iconic fashion history rundown of her “clearance bin” sweater.
But cerulean itself has far more than fashion history behind it. Blues—including cerulean—are some of the rarest colors in nature, and astonishingly difficult to create. And yet, throughout history, humans have tried, over and over again, to reflect the blue of a perfect sky.
As The Devil Wears Prada 2 brings Miranda Priestly back to screens, it’s worth asking what it takes to create a color that’s not just turquoise or lapis, but cerulean. It turns out that to make the right blue, scientists need elements from blue-hued cobalt to potassium and magnesium. And in pursuit of color and cash, a new study shows that some paint-makers may have tried to keep their cerulean chemistry a secret.
(This is the oldest blue pigment ever found in Europe.)
From Egyptian tombs to French canvas
“Blue came along fairly late as something that you could color things with,” says Kai Kupferschmidt, a science journalist and author of the 2022 book Blue: The Science and Secrets of Nature’s Rarest Color.
That’s in part because it is rare in nature. Humans have been painting in color for more than 60,000 years, but they often stuck with charcoal, red and yellow, Kupferschmidt writes. The first known blue pigments were ground up from the blue mineral azurite and used in graves in Turkey nearly 9,000 years ago.
Egyptian pharaohs imported the lapis lazuli, a deep blue mineral-filled rock from the Hindu Kush mountains in Afghanistan, at fabulous expense, but they also set out to make their own—the world’s first synthetic pigment developed around 3000 B.C. “One of the earliest examples is Egyptian blue or calcium copper silicate,” says Mas Subramanian, a materials chemist at Oregon State University in Corvallis.
The result was the brilliant blue glaze found on Egyptian grave goods and other pottery. The word cerulean originated from a Latin term, caeruleum, “used in ancient Rome for Egyptian blue pigment,” says Václava Antušková, a conservation scientist at the Chemical-Technological Laboratory in the National Gallery of Prague in the Czech Republic.
People also produced blues from plants like woad and indigo, both sold at fabulous prices. By the Renaissance, ultramarine produced from lapis lazuli was worth its weight in gold.
(Barbie pink may be Earth’s oldest color.)
What does blue signify?
Because blue raw materials were rare, the ancient world often didn’t infuse it with a lot of symbolic meaning, writes historian Michel Pastoureau in his 2018 book Blue: The History of a Color. It’s hard to make a color a symbol of something when you don’t have a paint for it, after all.
But as blue paints became available in antiquity, artists began to use blue symbolically. The Virgin Mary is depicted wearing a deep blue robe. Kali, the Hindu goddess of time and death is often depicted as blue. More recently, baby boys—and girls—have been associated with baby blue.
Blue has held a lot of meanings over time. Someone can have “the blues,” but a good time ahead can look like “blue skies.” Like the cerulean sky itself, Kupferschmidt says, “blue is like this huge canvas that's pretty empty. And so people have an ability, I think, to put associations on it and use it differently.”
The chemistry blues
While blue has been hard to pin down metaphorically, it’s even more difficult to produce. That’s in part because, unlike other colors, it is a very specific type of light that our eyes see in one particular way, says Subramanian.
Light travels as a wave, but wave patterns can be different sizes and frequencies. Colors are a result of light moving at different wavelengths or energy levels. Our eyes perceive an object as a particular color because that is the color wavelength reflected off it—the final wavelength left over that the object failed to absorb. When we perceive a rainbow from red to violet, we are seeing different wavelengths of light, with red being the highest wavelength and blue the lowest.
Absorbing a lower energy wavelength is easy, but reflecting it is hard. That’s what makes blue an issue, Subramanian says. To appear blue “a material must absorb lower-energy red/orange light while reflecting higher-energy blue wavelengths. Most compounds do the opposite.”
The color we now know as cerulean presents a particular problem, Subramanian says. “The challenge is not just absorbing red light but shaping the absorption profile so that a small portion of green is also attenuated without shifting fully into turquoise.” The best option, he says, is to start with the element cobalt. It naturally reflects the right kind of blue.
Creating cerulean
While the word is from Latin, cerulean was revived in the 18th century by the Swiss chemist Albrecht Höpfner, and used as a paint in the art world in the 1860s. This particular cerulean was a mixture of cobalt and tin, and billows in the blue clouds of steam and smoke coming off the train engines in Monet’s 1877 painting The Gare St-Lazare. The same cerulean colors the sky in the heads of two figures in Dali’s Couple aux têtes pleines de nuages from 1936.
As a conservation scientist, Antušková wanted to understand exactly what this cerulean was made of and how. Museums have to authenticate artworks, and paint chemistry is one way they do it, she explains. Sometimes, they have to remake the paint itself. “Reconstructions following historical recipes gave us valuable comparative material,” she says.
In a 2026 study, Antušková and her colleagues set out to re-create cerulean blue using different historical methods that were documented in the 1860s. They mixed together a chemical called potassium stannate with cobalt nitrate. The scientists also tried cobalt chloride with tin chloride and sodium carbonate. Both produced a chemical called cobalt stannate, which should, according to the records be cerulean.
It wasn’t. The resulting compounds came out in various shades of green. “Some samples were really dark, almost black with greenish hue, some of them were a bit greyish green,” Antušková says. Something was missing.
Analyzing scrapes of the paint from the historical paintings showed high magnesium levels, and when Antušková and her colleagues added magnesium to their mix, making magnesium cobalt stannate, they were rewarded with brilliant cerulean blue.
The missing magnesium could have been a trade secret, Antušková says. “The producers might have kept it as a business secret to prevent others from reproducing the pigments,” she says. But authors of the paint recipe books may have also just assumed readers already knew, or repeated mistakes without checking them.
The bluest blue
Today, cerulean blues are everywhere, and still made using similar ingredients.
And it still has an aura. When Pantone picked their very first color of the year in 2000, Kuperschmidt says, they picked cerulean (even though The Devil Wears Prada wouldn’t come out until 2006). “In fact, they called it the color of the millennium,” Kuperschmidt says. “That's a lot of marketing, of course, but it has that kind of standing.” It’s even the color of the United Nations flag, adopted as the color of peace.
Subramanian agrees that the hue remains special—high praise from a chemist whose lab synthesized the most intense blue ever created, YInMn blue, back in 2009. Cerulean blues are hard to make, but “easy to incorporate into plastics and coatings used in everyday items,” he says. “At the same time, their association with sky and water gives them a clean, calm, and widely appealing appearance.”
(Read our interview with Subramanian about the bluest blue discovery.)
Once pigments worth their weight in gold, blues like cerulean are now common in the objects around us, in plastic trash cans, tools, brand names, and “lumpy blue sweaters.” As Miranda Priestly notes, cerulean “represents millions of dollars and countless jobs” to create that specific color. It represents history, symbolism, and chemistry. And most of all, it reflects a deep desire to recreate the beauty of the natural world.
