Catherine made her first visit as Royal Patron of the Victoria and Albert Museum on Wednesday night in London, where she opened the new photography centre.
She paired the dress with new, floral hooped statement earrings from the label and teamed the frock with deep purple accessories, including a waist cinching belt and plum velvet pumps from Jimmy Choo.
Kate and Will chose a mental health summit as their first royal engagement after the Duchess' maternity leave.
The exhibit is titled Collecting Photography: From Daguerreotype to Digital and follows the medium's invention to the present day.
The couple were greeted by UK Health Secretary Matt Hancock as they arrived at the inaugural event, with crowds cheering as they caught a glimpse of the royal couple.
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HELLO! magazine reports that Kate was shown a pencil drawing by William Henry Fox Talbot, who has been referred to the "father of photography".
She added her own artistic touch to an original piece by Dairo Vargas, a contemporary fine artist from Columbia, who created a painting inspired by the day's events.
She also met with curators during her visit.
The next appearance for the Duke and Duchess of Cambridge and the Duke and Duchess of Sussex is expected to be at the wedding of William and Harry's cousin, Princess Eugenie, when she marries Jack Brooksbank on October 12.