Meghan Markle, 36, raised eyebrows with a shoulder baring dress at the Trooping of Colours parade last Saturday.
The wife of Prince Harry, 33, appeared on the Buckingham Palace as a member of the royal family for the first time.
Meghan, who was given the title of Duchess of Sussex upon her wedding to Prince Harry, is the newest member of the royals.
While not strictly against royal protocol, her choice of an off-the-shoulder dress contrasted with the likes of Kate Middleton, who always wear sleeves.
Speaking about Meghan’s custom-made Carolina Herrera dress on ITV’s Lorraine, fashion expert Mark Heyes called it “very unusual”.
“It’s very unusual to see a member of the royal household baring her shoulders,” he told Lorraine.
“Especially at a daytime event like that,” he added.
“It’s not that it’s not allowed, but maybe it’s a sign of things to come.”
Mark also referred to Meghan’s look as “fashion forward rule breaking”.
He also said Meghan was “looking beautiful”. “I thought she looked absolutely stunning,” he said.
Meghan’s choice of an off the shoulder dress was a nod to her wedding dress, which had a bateau neckline.
However, while her wedding dress showed off her neck and collarbone, her shoulder-baring gown went one step further.
This isn’t the first time Meghan has broken royal protocol with her style choices.
A month before her wedding to Prince Harry, Meghan broke one of the Queen’s royal style rules.
She braved bare legs during a public appearance, showing off her calves under a floaty dress.
The style move, while appropriate for the hot weather, broke the Queen’s protocol for how royals must dress.
The Queen’s ‘only’ rule with regards to what female members of the royal family wear in public is that they were clear nude stockings when they are on public duty.
The Queen’s royal rule was revealed by royal expert Victoria Arbiter in an interview with Insider magazine.
She told the publication: “You never see a royal without their nude stockings.
“I would say that’s really the only hard, steadfast rule in terms of what the Queen requires.”