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 Duchess of Sussex, 36, wore an olive green dress by the label, topped off with a wide-brimmed hat by Stephen Jones. "I remember the Battle of Britain being fought in the skies above us and we shall never forget the courage and sacrifice of that time", she added, referring to the decisive encounter between the air forces of Britain and Nazi Germany over southern England in 1940. Meghan wowed in a black cocktail dress which was teamed with a matching clutch and heels.
The outfit called to mind a similar ensemble that Kate wore to Meghan and Prince Harry's wedding day on May 19 - a pale yellow version - as well as the cream take on the look that she chose for Princess Charlotte's 2015 baptism. Meghan's signature face-framing tendrils were nowhere to be seen.
Just one day after attending the christening of Prince Louis, the royal family was out and about again to celebrate the 100th birthday of the Royal Air Force.
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He became a leading figure in the "Leave" campaign, painting a rosy picture of Britain's future outside the European Union. Many Brexit campaigners in her Conservative Party say she has betrayed her promise to pursue a clean break with the EU.
The couple met RAF servicemen and women, charity representatives involved in the RAF100 Appeal, RAF Veterans and Civil Service personnel involved in the Centenary celebrations.
Harry and Meghan's engagements will include official meetings with Taoiseach Leo Varadkar and President Michael D Higgins.
Meghan Markle has stunned in Ralph Lauren at the royal christening of Prince Louis.
The royal couple with also meet with young people interested in coding when they visit the startup hub Dogpatch Labs in Dublin 1.