When Camilla Parker-Bowles and Prince Charles tied the knot at Windsor Guildhall in 2009, only one attendee wore white.

Oddly, however, that person was not the blushing bride – it was the Queen.

The reigning monarch at the time was Charles’ mother and Camilla’s future mother-in-law, Queen Elizabeth II.

She’d been sitting in the second row of the civil ceremony, wearing an elegant white coat which she’d paired with a matching hat. Her headwear also bore several subtle flecks of yellow.

The Queen dressed down her largely bridal get-up with a pair of suave black gloves, a black handbag and black shoes.

Millions of onlookers watching the wedding live from the comfort of their living room sofas were perplexed at the time as to why the monarch had been permitted to wear the colour outfit that’s usually reserved for the bride.

This confusion was further exacerbated when Camilla, the actual bride, turned up to the church wearing a pale blue gown, rather than the traditional white dress many expected to see.

It turns out that royal dressers had recommended the Queen wear a cream number because Camilla had already chosen an oyster-blue outfit.

This decision was made in light of the fact that the wedding was both Charles’s and Camilla’s second. Also, given that she was 57 at the time, she reportedly sought a more appropriate, elegant look.

As many fans of the monarchy will be more than aware, Queen Elizabeth, meanwhile, previously bore a passion for wearing bold colours to public appearances. In fact, blue was widely considered her favourite colour, based on wardrobe studies cited by HELLO!, and apparently, she had a particular fondness for teal, cobalt and powder blue.

As such, after catching wind that Camilla was to wear a blue dress, rather than a white one, she selected her outfit accordingly so as not to clash with the mother-of-two.

This explanation contradicts the theory held by many fans of Princess Diana, who are adamant that The Queen wore white to show her support of her son’s first wife, who died in 1997.

“I love how Queen Elizabeth still honoured her after all those years,” one Diana-fan wrote on social media after footage of the royal wedding was re-shared.

Camilla and Charles had been romantically involved both before and during his marriage to Diana, who famously brought scandal onto the monarchy by telling the BBC: “There were three of us in this marriage, so it was a bit crowded.”

Queen Elizabeth had attended Charles and Camilla’s wedding, despite having hesitations as the Head of the Church of England, given that Camilla’s first husband, Tom Parker-Bowles, was still alive in 2005.

Though the monarch granted the couple permission to wed, so as not to directly ignore the religious teachings of her own church, she did not attend the subsequent legal service at St George’s Chapel.