Fendi Put Mermaid Hair on Kendall, Gigi, and Kaia Gerber

The mermaid hair trend really found its legs on Instagram before making it to a salon near you. (Studios like Bleach London had a lot to do with it as well, but if it wasn't for social media, it certainly wouldn't have spread so fast.) And now, the dye job that can be seen on nearly half of the millennials in New York and London has hit the runways in Milan. At Fendi's Spring 2018 show, hairstylist Sam McKnight dyed faux fringe various shades of blue and green and stuck them on top of the models hair almost like little hats. And you might be surprised to find out exactly how he created the cool hues.

The idea to go with various blue- and green- colored fake bangs came from the winged liner that makeup artist Peter Philips drew onto the models' lids. (He painted on Dior's Metalizer Metal Creme Shadow in Green Magnet, a forest green that's so dark and shiny, it looks almost black or gray, it's hard to tell really.) "During the makeup test to figure out the look for the show, Peter had an array of liner marks on the back of his hand that were all sorts of greens and blues," explained McKnight, "so we decided to dye the hair pieces in similar tones."

But creating the moody tones — which ranged from aquamarine to a deep, smoky green — wasn't as easy as slapping on some hair dye and calling it a day. To mimic the "Petrol-like effect" of Philips liner (Petrol, meaning the multifaceted not-quite-gray/not-quite-green/not-quite-blue of an oil slick), required some troubleshooting. "The secret is to add black and gray [dye] to the hair," McKnight explained.

After dyeing the hair pieces blue, green, or some mix of the two using neon, Manic Panic-like dyes, McKnight then dipped them into water that had been laced with black or gray hair dye. "The black or gray kills the neon that's in the dye. We discovered the trick when we did those pastel wigs for the [Chanel Cruise 2013] show at Versailles. The hair color we used for those wigs was too bright — the fabrics were not like that, they were more subdued — so we added drops of black and gray hair dye to water and dipped the wigs in." Five years later (the show was in 2012), McKnight brought back the technique for Fendi.

Underneath the fake pieces of fringe were low, side-parted ponytails that McKnight smoothed in place with his Modern Hairspray. "This [hairstyle] is quite a lot of things," said McKnight. "It's a fake undercut, it's a ponytail, it's like what we called a "neat and crop" in England, it's faux bangs..." Or, it's the high fashion version of mermaid hair, meaning this crazy hair color trend is most definitely here to stay.

For more fashion week beauty coverage, keep reading:

1. I Just Discovered the Easiest Contouring Trick Ever From Tom Pecheux at No. 21 Spring 2018

2. Pastel Makeup Looked Incredibly Sexy at Alberta Ferretti Spring 2018

3. The Best Manicures of NYFW Spring/Summer 2018

Look back at the past 100 years of hair color trends: