Kate, mother to Prince George, Princess Charlotte and Prince Louis, looked lovely in the long dress which she wore with a fashionable belt and with her long, dark hair down, over her shoulders.
This time around, the royal attended the inaugural Global Ministerial Mental Health Summit with Prince William wearing a frock she wore on last year's goodwill tour of Poland and Germany.
William and Kate were greeted by Health Secretary Matt Hancock.
The event was attended by political figures, experts and policy makers who proceeded to discuss issues such as how to deal with the stigma attached to mental health - a subject very close to the royal's heart.
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.
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"That's pretty much ruined the picture", William joked, via E!
During her maternity leave, Kate made a handful of public appearances largely for family events like brother-in-law Prince Harry's wedding to Meghan Markle and to mark the annual "Trooping the Colour" at Buckingham Palace. The initiative was started by Dixon Chibanda and teaches the women to become lay health workers.
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'.
The duchess first showed off the Emilia Wickstead dress on a trip to Germany previous year.