Meghan Markle, aka, the Duchess of Sussex, is something of a trailblazer as a royal. She has bucked royal traditions on everything from her choice of wedding cake to hugging members of the public. And her most recent act of rebellion is subtle, yet significant: She wore a dark mani.

On Monday, Markle made a surprise appearance as a presenter at the British Fashion Awards, wearing a one-shoulder, black Givenchy gown (she was presenting an award to Givenchy dessigner Clare Waight Keller). But it was her dark nail polish that ignited a frenzy on social media.

The dark-hued nails were shocking because they break royal protocol. As a royal, Markle has royal duties and therefore is expected to adhere to special — and specific — traditions and etiquette. Royal women are usually expected to wear natural makeup and nails, for example.

In fact, Grant Harrold, the former butler to Prince Charles, William and Harry, previously told CNBC Make It that dark nail polish is an "absolute no-no."

"It is the more natural look, it's not anything too bright," Harrold said of the protocol for fashion. "Same with nail varnish and makeup. It's always very natural, neutral, skin tone colors… you don't wear dark or black or gothic...They tend to opt to wear more sort of neutral colors, just not to draw attention to themselves, really."

Even the duchess's dress could be considered rebellious. Black is also seen as off-limits for clothing, because it is the color associated with royal mourning.

"Diana, Princess of Wales, on a couple of occasions, famously wore black," Harrold said. "So it's not written in stone that they can't, but it tends to be something they wear only when a member of the royal family or someone close to them has passed."

Markle, though, is paving her own way as a royal. At the British Fashion Awards, Markle said she picks what she wears not just based on how something looks, but also what the designers stand for.