She paired the outfit with nude heels and a pale lavender handbag. He added: "She was genuinely knowledgeable and excited about photography - particularly the history".
Prince William joked that the painting had been ruined.
Middleton, who gave birth to Louis on April 23, has made appearances at events, such as the RAF centenary, Trooping the Colour, and Wimbledon, but she's only just officially returned back to work.
This saw the couple break out into laughter along with the crowd.
The Duke and Duchess had a chance to speak to young delegates at the summit about the challenges and issues they face.
The project's founder Dixon Chibanda said: "Bringing our Friendship Bench to London helps show leaders and campaigners how we can all empower communities to tackle the global gap between mental health problems and the treatments available".
The duke and duchess were greeted by Health Secretary Matt Hancock
The Erdem dress worn by the Duchess last night was originally shown on the catwalk as part of the British brand's SS18 collection inspired by the Queen's 1950s wardrobe.
She has expressed her love for photography and has taken portraits of her children Prince George and Princess Charlotte in the past.
In the lead-up to hosting a Marathon for Mental Health in the United Kingdom in 2017, Prince Harry revealed he'd sought counselling in his early 20's after two years of what he labelled "total chaos" following the death of his mother, Princess Diana.
Last month Prince William opened up about the emotional toll of witnessing suffering during his time as an air ambulance pilot.
Erdem has become somewhat of a go-to designer for members of the royal family, with the label being the favourite to design Princess Eugenie's wedding dress.
We shot ourselves in the foot, says Hearts boss Craig Levein
The team put in a good performance again. "I said it before the game and I wasn't telling lies - Hearts are where they are on merit".