On Saturday, Queen Elizabeth II abandoned the royal protocol of the monarch not being allowed to wear an outfit more than twice.

The Queen appeared for her annual official birthday celebration, "Trooping the Colour," in a Lilac dress coat and hat decorated with yellow flowers that she had already worn twice. According to royal experts, it's the monarch's subtle way of showing that she is still in mourning after the loss of her husband Prince Philip in April this year, reports The Telegraph.

She had worn the exact same outfit at the State Opening of Parliament just a few weeks earlier. It was also the third time she wore the coat itself in public.

An old royal custom dictates that widows can wear only black mourning clothes for the first two years after their husbands' passing. After this period, they are expected to wear understated colours such as grey and lilac as their 'second mourning' clothes.

Matthew Storey, curator at Historic Royal Palaces, explained to the outlet: "Mourning dress has been part of European royal culture for centuries, but it reached its peak in the 19th century with the influence of Queen Victoria, who set a standard for the rest of society to follow."

Queen Victoria, the longest-reigning British monarch before the current Queen, had entered a state of mourning after the death of her husband Prince Albert in 1861 and wore black for the remainder of her life. She avoided public appearances and rarely set foot in London in the following years, which earned her the nickname "Widow of Windsor."

Though no one after Queen Victoria mourned like that, the royal widows were still "required to wear black, then either white or mauve, for at least three years before being able to return to richly coloured clothing."

Royal experts believe that the reigning Queen is putting a modern spin on the centuries-old tradition by following the second mourning colour profile for official royal events such as Trooping the Colour.

Queen Elizabeth II
(FILES) In this file photo Britain's Queen Elizabeth II smiles as she attends day three of the Royal Ascot horse racing meet, in Ascot, west of London. Photo: AFP / Adrian DENNIS