The British royal family gathered at Buckingham Palace in London on Tuesday to watch a flypast celebrating 100 years of the Royal Air Force.
Harry, the Duke of Sussex, also wore his No 1 RAF uniform, with RAF Regiment shoulder flashes, in recognition of Honington - the home of the RAF Regiment - and also for his role as Honorary Air Commandant of the base. Wearing a pastel blue coatdress, the mother of three accessorized with a small matching clutch, beige heels and a fascinator by Sean Barrett.
Meghan gave a nod to their host country upon arrival, wearing an emerald green dress by Givenchy, her wedding dress designer, as she and Harry, dressed in a suit, departed the plane.
Meghan and Harry are officially in Ireland.
The Duke and Duchess of Sussex began their first official worldwide visit together when they arrived in Ireland on Tuesday.
"I think it looks very smart".
Collier told MailOnline: "The bun is quickly becoming the Duchess's signature look, so it's no surprise to see her sporting this style at today's ceremony".
The Duke and Duchess will also visit DogPatch Labs, a co-working space for technology start-ups, where they will meet children and young people taking part in workshops, and talk to female tech entrepreneurs.
On Wednesday, their day will start with an official meeting with Michael Higgins, the president of Ireland, before heading to Croke Park, where they will view an exhibit that includes artifacts from the Bloody Sunday massacre.
The couple looked loved up a day previous on Monday, when they attended a fly-past marking the centenary of the British Royal Air Force.