Kate wore a Day Birger et Mikkelsen coat at Zara's wedding (Image: GETTY)
The Princess of Wales, who married three months earlier herself at Westminster Abbey, surprised fans when she stepped out wearing a cream/gold coat at the wedding.
Usually, female guests do not wear white or any similar colours or shades to a wedding out of respect for the bride.
Style expert Susie Nelson, the founder of Modes and More, spoke exclusively to Express.co.uk to explain why Kate wore this coat to Zara's wedding.
She said: "I think the issue is that people perceive colours in various ways and fabric reflects light in different ways, so whilst some royal watchers were convinced that Kate's coat was cream, she actually wore pale gold to Zara's wedding.
Kate looked beautiful at Zara's wedding (Image: GETTY)
"White or cream would have been a breach of royal protocol. The coat was by DAY Birger and Mikkelsen, and she had previously worn it to the wedding of Laura Parker-Bowles.
"Like Princess Anne, Kate is a fan of sustainable dressing and enjoys re-wearing her favourite clothes. The approximate value of the coat is between £800 and £1,000."
According to style expert Leroy Dawkins, a celebrity stylist and royal expert, Kate's coat was a sustainable choice.
He claimed: "In July 2011, Kate Middleton, the then-Duchess of Cambridge attended the wedding of her cousin-in-law Zara Phillips and Mike Tindall at Canongate Kirk in Edinburgh, wearing a DAY Birger et Mikkelsen light biscuit coloured coat for the ceremony.
Kate and William tied the knot three months before (Image: GETTY)
"The Princess first debuted the coat at the marriage of Laura Parker-Bowles (Queen Camilla’s daughter) to Harry Lopes in 2006.
"The coat then made its second outing when the then-Duchess wore it to the marriage of Mel Nicholson and Oli Baker at St. Andrews in 2010.
"The last sighting was at Zara Phillips and Mike Tindall's wedding in 2011."
Aimee Howlett, a jewellery and trends expert at Diamond's Factory exclusively told Express.co.uk: "Traditionally, there are no specific royal protocols regarding the choice of clothing for attending weddings and many members of the Royal Family, including Kate Middleton, often adhere to customary etiquette and choose attire appropriately for the occasion.
Kate previously wore the coat at Laura Lopes's wedding (Image: GETTY)
"The decision to wear cream to Zara's wedding would likely be a personal choice made by Kate based on her own style preferences.
"Wearing cream to a wedding is often a popular choice as it can be an elegant and sophisticated colour. Traditionally, it is advised to avoid wearing white, as it is typically reserved for the bride."
She opined: "I don't believe this was a break in royal protocol, as all throughout the history of the Royal Family, women have worn cream and soft yellow as wedding guests.
"In fact, it is custom for the bridesmaids to wear the same ivory as the bride, and this seems to also be acceptable for royal guests, despite the fact that amongst general society it is thought of as taboo to wear cream of white as a wedding guest.
"Under the coat, she also wore a Collette Dinnigan dress. She previously wore the dress in 2006 to the Parker Bowles–Lopes wedding, and again in 2007 to the 'Young Stalin' book launch.
"Kate is a big fan of recycling her wardrobe and repeating her outfits, being an advocate of Sustainable Fashion.
"As witnessed by her appearance wearing a rented green dress from HURR to the Earthshot Prize last year, celebrating groundbreaking solutions to repair our planet."