Why Kate Middleton's Unexpected Teal Ensemble Was the Perfect Shade for St. Patrick's Day

Lots of thought goes into Kate Middleton's outfit choices — and her St. Patrick's Day ensemble for her holiday debut as the new honorary Colonel of the Irish Guards was no exception.

Keeping with a long-running tradition, Kate and Prince William stepped out to visit the Irish Guards at the St. Patrick's Day Parade in Aldershot on Friday. While the Princess of Wales usually marks the event wearing green, the color most closely associated with the holiday, she made a surprising decision to wear a teal Catherine Walker & Co coat dress with matching accessories for the occasion this year.

While the color choice was certainly unexpected, there's a clever reason for it. Princess Kate's ensemble perfectly matched the plumes worn in the bearskin caps by the 1st Battalion Irish Guards. The plume being St. Patrick's blue was selected because that is the color of the mantle and sash of the Order of St. Patrick.

In June, blue was the color of choice for the royal family as they kicked off Queen Elizabeth's Platinum Jubilee celebrations with Trooping the Colour. Queen Elizabeth stepped onto the Buckingham Palace balcony wearing St. Patrick's blue, a nod to the original symbolic color of Ireland which can also be representative of sovereignty. Many royal women also opted for shades of blue to coordinate with the monarch.

Karen Haller, behavioral color psychologist and The Little Book of Color author, previously spoke to MyLondon about the royal family's typical blue outfits, noting it shows that "they are coming together as a family, representing themselves as a cohesive unit."

"When it comes to color psychology, darker blues communicates you are in a position of authority, trustworthy, reliable and can be depended on," Haller told the outlet. "You have a sense of duty and take that seriously with committed focus."

For the St. Patrick's Day parade, Princess Kate also chose meaningful accessories. In addition to sporting diamond and emerald earrings, giving her outfit a touch of classic green, she sported a special shamrock brooch featuring a small emerald in the center.

The brooch belongs to the Irish Guards and is loaned to the royal women who are associated with the regiment, according to The Court Jeweller. Before Kate, it was worn by Princess Anne and the Queen Mother.

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Kate's coat dress also mirrored the Irish Guards' uniform with four buttons down the front then a gap before four more buttons.

Both Prince William and Princess Kate gave speeches at the event, highlighting the transition in the royal roles. King Charles recently named his daughter-in-law as the new honorary Colonel of the regiment, and she takes over the role from Prince William, who is now the equivalent of the Welsh Guards to go with his Prince of Wales title.

The Prince of Wales said, "Whilst I am extremely happy to be here celebrating St. Patrick's Day with you once again, I'm incredibly sad to be giving this speech, as it really does mean my time as your Colonel has come to an end. It has been one of the great honors of my life to hold that title; I'm proud of everything it stands for — just as I'm immensely proud of every single one of you."

He also poked some fun at himself and the tradition of toasting the holiday with a pint of beer.

"On this, my 13th and final St. Patrick's Day as Colonel, not only am I saddened that I'll likely fail once again to finish a pint of Guinness… but I am also sad that I won't in future see more of you, the families, whose unwavering support enables these outstanding men and women to do what they do," Prince William, 40, said.

Kate, who her husband referred to as "Colonel Catherine" in his speech, said her new role was a "true honor."

"I really couldn't be prouder to stand in front of you here today. It really is a true honor to be your Colonel. I am here to listen to you, to support you, and to champion you in all you do – this is a responsibility I do not take lightly," she said.

"I look forward to spending more time with you and your families, seeing your commitment to duty and service in all you do. Before I close, however, I want to thank Colonel William. He has always talked about his fierce pride for this Regiment. I know he will continue to support us all in the work we do," Princess Kate, 41, continued. "Finally, I would like to wish you all, a very happy St. Patrick's Day."

Kate took part in the traditional distribution of shamrock sprigs, which the guards pin to their tunics. Also getting a sprig of the Irish bloom was the regimental mascot, Irish wolfhound Turlough Mor, who is known as Seamus.

The parade ended with Princess Kate receiving a royal salute as the troops marched past.

Following an inspection of the troops, Prince William and Kate sat for official Officers' and Sergeants' Mess photographs before they headed to the dining hall. There, they joined the Irish Guards in raising a toast with a glass of Guinness.