While Kate Middleton is unlikely to ever divulge the secrets of her daily routine, the Duchess has certainly fine-tuned her signature beauty look to perfection since joining the royal family. There’s the glossy brunette blow out — dried with a slight wave and plenty of volume — the glowing complexion (reportedly thanks to Trilogy’s cult favorite and budget-friendly Rosehip Oil), strong brows, and a subtle smokey eye. It’s a classic look, and one that’s worthy of one of the most photographed women in the world.

Just as it’s rare for us to see the Duchess testing out a bold red lip instead of the neutral pink she favors, Kate similarly steers clear of bright shades on her nails: in fact, she’s never been photographed at a public engagement sporting anything other than a barely-there manicure in the lightest shade of blush (though she sometimes plays things a little less safe — shock horror – when it comes to her pedicures). The reason? As with many of those niggling questions about royal behavior, it’s all a simple matter of etiquette.

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Kate sporting her favorite nail polish, Essie’s Ballet Slippers. (Photo Credit: Getty Images)

Just as the royals have to ensure that their outfits obey a strict set of sartorial rules, their beauty choices are similarly dictated by protocol. According to OK! magazine, certain well-placed sources have let it slip that wearing fake nails or colored nail polish is considered “vulgar” for royal engagements.

Instead of bright colors, the royals are encouraged to favor a neutral hue on the nails. Queen Elizabeth has famously favored Essie’s pale pink Ballet Slippers polish since 1989 — her hairdresser reportedly described it as “the only color Her Majesty would wear”. For her wedding day in 2011, Kate took her cues from her grandmother-in-law, opting to wear the same shade when she tied the knot with Prince William. Since then, she’s stuck close to the same formula: a neat manicure in a barely-there color.

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This post was written by Katie Rosseinsky. It originally appeared on our sister site, Grazia Daily.