The Mental Health Summit is taking place over two days and will see political figures, leading academics, and policy makers from around the world come together to support better mental health for all.
Middleton attended her first solo royal engagement since her son's birth last week, when she visited the Paddington Recreation Ground in Ewhurst, England to spend time with local school children.
They are hoping to put mental health on an equal footing with physical health. The meeting is set to help countries work together to deal with stigma attached to mental health and address other issues to do with mental ill-health.
The Duchess previously wore the dress, named the "Kate", during an appearance in Hamburg, Germany last summer when she and William took a royal tour with children Prince George and Princess Charlotte.
The royal couple arrived to attend multiple presentations on case studies of active mental health programs and also met with young people to speak about their experiences with mental health.
William ushered his wife first and said: "My wife, she's the arty one!"
Kate, 36, who studied History of Art at St Andrews, where she first met her husband, laughed and took the brush from Mr Vargas, asking him: 'Where can you hide it?' Mr Vargas pointed to a shoulder area and Kate made her brushstroke - as William joked: 'It's nearly ruined the picture'.
Prince William then teased his wife, saying "It's nearly ruined the picture", after she added a white stroke to the painting, but later joked: "no, this is probably how you ruin a picture", as he added his own mark.
She wore a handsome purple dress from Emilia Wickstead for the outing teamed with a pair of nude heels.
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