Why was everyone wearing white at that Buckingham Palace dinner?

The dress code was white tie, not white gown, but when Queen Elizabeth celebrated diplomatic ties with the U.S. at Monday night's state dinner, she was far from the only woman in attendance to don a white dress.

In fact, almost all the leading ladies who attended the banquet — including First Lady Melania Trump, Catherine, Duchess of Cambridge and Camilla, Duchess of Cornwall — stuck to alabaster designs.

But why?

For those who aren't accustomed to attending white-tie events (i.e. the vast majority of us), the fashion rules for the most formal of all dress codes require men to wear a black tailcoat and trousers with white shirt, vest and bow tie.

But for women, there are no limitations when it comes to color, only that they wear floor-length gowns and appropriate accessories.

So was this merely a fashion coincidence?

Maybe.

For the royal women, the choice simply makes sense. After all, the queen, the Duchess of Cambridge and the Duchess of Cornwall had boldly colored and complex items to drape over their otherwise neutral gowns, like bright blue sashes, order insignia, badges and other marks of distinction.

Perhaps they gave the first lady of the United States a heads up about the coordinating fashions or, just as likely, Trump felt that her stark white Dior gown was fitting for both the occasion and the season.

In fact, when former first lady Michelle Obama attended a state dinner at Buckingham Palace in 2011, hosted during the same time of year, she and the queen were also perfectly color matched in pale designs.

Of course, the color white has been having a moment in recent years — especially when it comes to political events.

Historically associated as the color the suffragettes wore while fighting for women's right to vote in the early 20th century, democratic congresswomen made a similar sartorial statement in 2017, when the president first addressed congress.

As for Monday night's banquet, that wasn't the first time the first lady or the Duchess of Cornwall wore the summery shade during the highly anticipated state visit.

When Trump first stepped out onto the palace lawn earlier in the day, she did so in a white Dolce & Gabbana dress, and she was greeted by Camilla, also in white.

And it should be noted that not all women who attended the banquet stuck to the unofficial color of the evening.

First daughter and presidential adviser Ivanka Trump wore a cornflower blue gown to the event, while her sister, Tiffany, wore red, and Britain's outgoing prime minister, Theresa May, wore black.

Regardless of the colors worn, everyone looked picture-perfect for the royal engagement.