Kate Middleton's pale blue dress matched the Duchess of Cornwall's

A curious tradition had the Royal Duchesses coordinate the color of their outfits

One of the most spectacular events held to celebrate the Queen's official birthday celebrations is the legendary Trooping the Colour.

Trooping the Colour is a ceremony performed by the regiments of the British and Commonwealth armies, and since 1748, it has marked the official birthday of the British sovereign.

The regiments honor the Queen, as do her subjects, and among them were the Royal Duchesses of Cambridge and Cornwall.

Camilla, Duchess of Cornwall, and Kate Duchess of Cambridge participated in the parade, riding together in an open horse-drawn carriage, and wearing matching outfits, reported Harper's Bazaar.

Prince William and his father, Charles, Prince of Wales, and Heir Presumptive to the throne accompanied the celebration on horseback since they are both royal colonels of the Household Division, and their wives shared a carriage.

The two women were elegantly dressed in very different outfits but made in the same shade of pale blue, with hats to match. According to tradition, the Duchesses were wearing the same color as the Queen, but in a lighter tone.

Prince Harry and Duchess Meghan rode in the carriage behind Duchess Kate and Duchess Camilla. The Duchess of Sussex broke the unspoken rule and was exquisitely dressed in a Carolina Herrera off the shoulder suit, and wore a flirtatious pink 'fascinator' on her dark locks.

Read more about the Queen's birthday on our Twitter account @amomama_usa.