The coat dress was actually a very pale yellow at her brother-in-law's nuptials, not cream, and the sleeves on the wedding look had buttons.
The event was the first time the Duke and Duchess of Cambridge appeared publicly with all three of their children.
The chapel holds tremendous significance for the royal family: Louis' elder brother Prince George was also christened there, and the chapel was where the late Princess Diana's body was laid before her funeral.
Louis, the 11-week-old youngest son of the Duke and Duchess of Cambridge, is being baptised by the Archbishop of Canterbury on Monday afternoon in the Chapel Royal, St James's Palace in London.More news: Belgium defeats Brazil to reach World Cup 2018 semi-finals
Prince Harry and Meghan were also in appearance, along with Kate's parents Carole and Michael Middleton.
Louis wore the replica of the royal christening robe, first used in 2008.
Hours before the ceremony, Prince Louis' godparents were revealed in an official statement from the palace. Guy Pelly has been a longtime friend of the Kate and Will and Harry Aubrey-Fletcher went to Ludgrove School and Eton with Prince William. As after George and Charlotte's baptisms, guests will be served slices of the duke and duchess's wedding cake.
Prince Louis was born on April 23 and is fifth in line to the British throne.
Hannah Carter, the wife of Robert Carter, is an old friend of Will and Kate's. (Charlotte's ceremony took place at St. Mary Magdalene Church, in Sandringham, instead.) Kate Middleton herself also received communion at the Chapel Royal just before her wedding to Prince William.