The 10 October marks World Mental Health Day, which Kate, William and Harry have shown an interest in for the past couple of years, as part of their Royal Foundation charity. Vargas said he hoped to have other attendees also contribute to the painting to symbolize how everyone can lend a helping hand in tackling mental health issues.
The pair attended the first Global Ministerial Mental Health Summit, an initiative that's been launched to raise awareness of mental health issues around the world.
Another policy up for discussion is a declaration committing to putting mental health on an equal footing with physical health.
It's the royal couple's first joint public duty since Kate gave birth to Prince Louis on 23 April.
While viewing the painting, the artist invited the royals to add to the piece, with Prince William nominating his wife as "the arty one", owing to her art history degree. At the summit, William and Kate visited a stand run by the non-profit organisation United for Global Mental Health, where they heard about the work of Dixon Chibanda, who started an initiative in Zimbabwe teaching elderly women the skills to offer therapy sessions on "friendship benches".
"That's pretty much ruined the picture", William teased his wife before poking fun at himself.
'You are allowed to go anywhere, ' encouraged Kate, who was wearing a £1,500 bespoke lavender Emilia Wickstead dress - previously seen on the couple's 2017 tour to Germany - with £510 Gianvito Rossi nude heels.
The Cambridges have two older children - Prince George, five, and Princess Charlotte, three.
Last month Prince William opened up about the emotional toll of witnessing suffering during his time as an air ambulance pilot.
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