These aren't the only Royal guests, if you want to see all of them and what they wore. Check this link. These aren't in alphabetical order.
The Duchess of Cornwall wore a pale blue dress, repeat a wind-blown hat, and is wearing the Dame Grand
Cross of the Royal Victorian Order. Princess Letizia wore a gray lace dress, a gray floral hat, and the Dame Grand
Cross of the Order of Charles III. Crown Princess Mary of Denmark wore a white dress with floral prints, a blue veil/floral hat, and the Order of the Elephant. Princess Mabel wore a black benie and a black dress with a single white sleeve. Crown Princess Masako of Japan wore a white dress with long sleeves and a turtle neckline, and the Order of the Precious Crown. Princess Mathilde, Duchess of Brabant wore a pink dress with long sleeves, a large pink hat, and the Order of Orange-Nassau.
Crown Princess Mette-Marit of Norway wore a blue printed floral dress, a flower head piece, and the Order of Orange-Nassau. Hereditary Grand Duchess Stephanie of Luxembourg wore a white dress with a white coat over top, a brown hat, and the Order of Adolphe of Nassau. Crown Princess Victoria wore a beige dress with a giant hat, which included flowers and feathers; and the Royal Order of the Seraphim
Sophie, Hereditary Grand Duchess of Liechtenstein wore a green dress and a green head piece and the Order of Merit of the Principality of Liechtenstein. Princess Aimee wore a gray/green maternity dress (she's expecting for the 3rd time!) and a orange hat Maxima has worn. Princess Irene wore a white lace dress,a matching hat and the Order of the Netherlands Lion.
Princess Laurentien wore a red hat, a red outfit and the Order of the House of Orange. Princess Marliene wore a pink dress and a pink hat to match.