Celebrity stylist Rochelle White told FEMAIL: 'Kate's latest look is something different from her usual style.
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.
During the event, William aimed a good-natured ribbing at his wife after Middleton, who studied art history at the University of St. Andrews in Scotland, added a brushstroke to artist Dairo Vargas' painting.
She paired the ensemble with black velvet pumps, a box clutch and handsome and unique pearl and diamond earrings.
This saw the couple break out into laughter along with the crowd.
The duke and duchess were greeted by Health Secretary Matt Hancock
Its eye-catching lilac colour looks divine on the mum of three, who returned to her royal duties following a six-month maternity leave last Tuesday.
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 Duchess chose a dress by Emilia Wickstead, wore previously during her 2017 royal tour of Germany.