By Alexandra Whittaker
Updated Apr 19, 2018 @ 2:15 pm

Prince Harry and Meghan Markle have gone glam for a night out exactly one month before they say "I do."

The engaged couple looked positively glamorous as they came out to Foreign Secretary Boris Johnson's reception on Thursday evening to champion women in the Commonwealth.

Markle wore a "Jackie O" Black Halo dress ($375; with an asymmetrical neckline, pairing it with a small black clutch and high black-tie heels. She looked as polished as ever, but her outfit did technically break a royal fashion rule.

Meghan Markle lead
Credit: Chris Jackson

As etiquette experts told InStyle last month, wearing black to formal events is usually a no-no.

"Generally it is thought that black is not usually worn unless in mourning, although Diana Princess of Wales did occasionally wear it for evening functions, and The Duchess of Cambridge has been known to do so," The English Manner's Alexandra Messervy said.

Even with that rule in place, choosing to wear black isn't really that big of a deal nowadays.

"I note that there was a bit of an outcry recently when Ms. Markle wore black for a daytime event, but in the modern age I don’t think that it is ‘frowned’ upon so much at all, and indeed funeral codes often state 'colors/no black," Messervy added.

Prince Harry, on the other hand, kept things simple in a dark blue suit. Both Harry and Meghan have much to celebrate right now. They're a month out from getting married, and on Thursday, they were both named to Time's list of the 100 Most Influential People of 2018. For the honor, Markle's friend Priyanka Chopra wrote about what she's like in person.

"With her, what you see is real, and what you get is a relatable young woman with her heart and mind in the right place, Chopra wrote. "Her compassion, evident in the causes she supports, and her drive to break down stereotypes—obvious in her actions—will connect her to a generation in much need of kindness."

Elton John had equally kind praise for Harry.

"Prince Harry has that rare ability to walk into a room full of strangers and make everyone feel comfortable and at ease," John wrote. "As he has grown in maturity, I have watched him take on these causes with the remarkable skill to see and communicate how it truly feels for the people he is trying to help."