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Kate Middleton, Duchess of Cambridge

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Kate Middleton brightened up the day at an otherwise rainy 90th birthday celebration event for Queen Elizabeth II Sunday. And not just because of her sunny smile.

Accompanied by husband Prince William, 33, and his brother Prince Harry, 31, the Duchess of Cambridge attended the Patron's Lunch in The Mall, a Buckingham Palace street party, which featured a massive picnic for some 10,000 members of the public who bought tickets for 150 pounds ($214) apiece. The event in London marks the monarch's patronage of more than 600 charities and organizations.

Kate, 34, turned heads with a striking, colorful outfit—a short sleeve, knee-length cornflower blue Roksanda Ilincic "Marwood" color-block wool-crepe dress, which featured a thick coral stripe across the waist and a vertical pastel pink stripe down the side. The designer is one of the duchess' favorites. Kate paired the look with nude pumps and a matching clutch.

Kate Middleton, Duchess of Cambridge, Prince Harry, Prince William

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The royals, including the queen, who was dressed in a hot pink dress and matching hat, and husband Prince Philip, 95, took part in a parade, during which they stood in the back of vehicles and waved at onlookers.

@thepatronslunch A dream come true - what an historic day! #Queenat90 ??????????

A video posted by Lydia Jones (@liddystardust) on

#theroyalfamily #queens90thbirthday #thepatronslunch #kensingtonroyal #thequeen #williamcatherineharry

A video posted by Louise Wellemeyer (@louzim) on

Nice spot of lunch #thepatronslunch #queens90thbirthday #themall #picnic #alittlebitofrainneverhurtnobody #sobritish

A photo posted by Amy Nankervis-Smith (@amy.nankervissmith) on

Kate, William and Harry also stepped down to talk to guests.

Kate Middleton, Duchess of Cambridge

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A photo posted by Darren Bradshaw (@dazb1) on

Many of the attendees put on ponchos when it began raining. Guests were not allowed to bring umbrellas.

"Thank you so much for showing that the Great British public doesn't let a little rain spoil a good day out," William said in a speech. "It means so much to see everybody here today."

The obligatory photo #BruntsOnTour #PatronsLunch

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"Granny, thank you for everything you have done for your family," he added. "We could not wish you a happier birthday."

The queen also spoke at the party.

"I much appreciate the kindness of all your birthday wishes, and have been delighted and moved by the many cards and messages I have received," she said. "How I will feel if people are still singing 'Happy Birthday' to me in December, remains to be seen!"

The event marked one of several marking the monarch's birthday, which she first celebrated in April. Kate and William brought their children Prince George, 2, and Princess Charlotte, 1, Friday to another birthday bash, the Trooping the Colour parade, during which their little girl made her first Buckingham Palace balcony appearance.