Chelsea Calkins, who keeps tabs on runway fashion in Paris, had dyed her hair various colours —pastels, full bleaches, shine bands around the middle — but eventually, each style got too popular back home in Hayes Valley.

And then she heard about grey. “You just never see it,” said Calkins. “My mom colours her hair. My grandma colours hers. All my aunts colour theirs. No one has gray hair.”

That sold her on it.

Last year, she went to Grasshopper Salon in SoMa and left with a full head of silver.

In a city that seems to obsess over youth, an odd phenomenon has been spotted — young people, mainly in their 20s, dyeing their hair grey.

And it’s showing up on everyone from pop-culture celebrities like Rihanna and Kelly Clarkson to fashion writers like Tavi Gevinson and the model Daphne Guinness.

In the Mission District, Amber Jahn, a 27-year-old stylist at the Pretty Pretty Collective, was skeptical when a client first asked for grey hair.

“I had to think about it for a second just to remember, ‘Oh yeah, that’s possible.’ I was like, are you sure you want to go gray?” she said. “It’s quite a process.

You basically have to lift all the pigment out of the hair because gray is such a cool tone.”

Gray is known as the hardest colour for a stylist to make. The bleaching process is extreme.

Any yellow left in the hair will make the gray look dingy.

So it can take two to three long trips to the salon for bleaching, especially for someone with darker hair. Then the stylist adds toners (blue to balance out yellow, etc.), totally depending on the hair’s natural colours. The result is a shimmering silver. “It’s almost like powdered wigs.

How trendy was that at some point?” Jahn said. “A gray streak is cred.”

At Grasshopper Salon, April Miles, 32, said she’s been getting a lot more requests for gray hair.

She thinks it fits into a broader appreciation for transgression in San Francisco. “We live in a place where people aren’t afraid to do things that are different — this is like gender-bender, but with age.”

Calkins has found that her friends think the grey is cool. It surprises people, and they’re often a little jealous. Though her mom had a different take.

“My mom just thinks it’s crazy, ‘cause she tries so hard to cover it up. I like to make sure that my mom never likes my hair.”