Princess Diana was criticized by Prince Charles for wearing a black dress to her first royal engagement

• Princess Diana was told off by Prince Charles for wearing black to her first royal engagement.
• It is customary for British royals to only wear black clothing during periods of mourning.
• "It was a horrendous occasion," Diana revealed to Andrew Morton in secretly recorded tapes.

Princess Diana revealed that Prince Charles criticized her for choosing to wear a black taffeta dress to their first royal engagement as a betrothed couple.

Diana spoke about the incident during secretly recorded interviews in 1991 for her biography "Diana: Her True Story" by Andrew Morton, which was revised in 2017 and titled "Diana: Her True Story - In Her Own Words."

The exchange took place before the couple attended a fundraising concert at Goldsmiths Hall, London in aid of The Royal Opera House on March 9, 1981.

"I remember walking into my husband-to-be's study, and him saying: 'You're not going in that dress, are you?' I replied: 'Yes, I am'. And he said: 'It's black! But only people in mourning wear black!'" Diana reflected.

Members of the British royal family must abide by many style rules and protocols, which include not wearing the color black as it is traditionally reserved for funerals.

"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," Alexandra Messervy, Chief Executive of The English Manner etiquette training provider, told InStyle.

Diana was only 19 when she became engaged to Prince Charles, 32, and was used to following the same fashion trends as her peers.

"I thought it was OK because girls my age wore this dress. I hadn't appreciated that I was now seen as a royal lady," she explained in the audio tapes.

The black satin dressed that immediately captured the public's attention was designed by British duo Elizabeth and David Emanuel, who also designed Diana's striking wedding dress in 1981.

Elizabeth Emanuel previously told British Vogue that the black dress marked a cultural moment for the princess, converting her from a teenager to a public figure. "We witnessed the birth of a fashion icon before our very eyes," she said.

"We didn't even consider whether the dress was right for the occasion, we just thought she looked fabulous," she added.

On the tapes, Diana sheds light on how overwhelming her first event was and how unfamiliar these royal customs felt to her. "It was a horrendous occasion," she told Morton.

"I didn't know whether to go out of the door first. I didn't know whether your handbag should be left in your left hand or your right hand. I was terrified really - at the time everything was all over the place," she explained.

It is no surprise that Diana's so-called "revenge dress," worn to Vanity Fair's 1994 fundraising gala, has since become such an iconic look.

Diana wore the black off-the-shoulder Christina Stambolian gown on the same evening that a documentary aired on national television in which Charles admitted he had been unfaithful to her.

The dress defied royal rules for many reasons, but its color may have been a nod to this early encounter, showing that she was no longer dressing for her husband or the royal family's approval.