- Queen Elizabeth II has become Britain's longest-reigning monarch
- Almost 1,500 officers and men took part, with 200 horses and over 400 musicians.
London (CNN)At 90, Queen Elizabeth II is getting bolder.
In grand celebrations to mark her milestone birthday, the Queen on Saturday sported a daring neon green outfit as she greeted the public, impossible to miss in her horse and open carriage, even past the thousands of viewers and hundreds of marching officers.
Celebrations began with the traditional Trooping the Color, a spectacular display of more than 1,400 officers and men in their famous red jackets and black bearskin hats, with 200 horses and more than 400 musicians.
The whole royal family -- including young Prince George and Princess Charlotte -- joined the nation's longest-serving monarch on the balcony of Buckingham Palace for a wave to the people and an exhilarating flypast.
Hurricane and Spitfire aircraft used in the 1940s Battle of Britain took part in the flypast, capped off with the air force's famous Red Arrows flying with red, white and blue smoke in the trail.
The national anthem, "God Save the Queen," was performed, as were several songs on the Scottish bagpipes and a repertoire of other marching band songs.
The monarch is technically the head of Britain's armed forces, and would traditionally lead an army into war. The parade gives the Queen a chance to review and approve her army.
In her younger years, the Queen would ride side-saddle on horseback to greet the public.
Her choice of color for Saturday's outfit triggered a social media storm, with Twitter users celebrating the monarch's fashion sensibilities with "neon at 90."
The Queen's real birthday is on April 21, but official celebrations always take place on June 10, and often for a whole weekend.
CNN's royal correspondent Kate Williams said security experts would consider the event a nightmare, with the monarch riding in an open carriage very close to the public and with no searches of the thousands of people who turned up to mark the occasion. Police have warned about crowded situations
"She's not having bulletproof vests, or going behind glass," Williams said, adding the Queen wanted to be close to the people.
The Queen was shot at with blanks in 1981 while on horseback, by a 17-year-old with a replica gun. The annual celebration has otherwise gone on without any major security concerns.
Before the festivities, a new photograph of the Queen and the Duke of Edinburgh, who turned 95 on Friday, was released to commemorate their joint birthdays.
The pair attended a thanksgiving service at St. Paul's Cathedral on Friday, where they were joined by other members of the royal family and politicians, past and present.
Readers at the service included renowned broadcaster and naturalist David Attenborough and British Prime Minister David Cameron.