According to the media:
Her Majesty Queen Máxima wore a fuchsia-coloured dress made of dupion silk with boot-neck and 3/4-sleeves, from the Belgian designer Edouard Vermeulen (House of Natan).
The hat was a prune-coloured one made from parabuntal and comes from Queen Máxima's favorite milliner, the Belgian Fabienne Delvigne. The matching cape however, is an affordable one from Zara, to be found in any high street in the bigger Dutch cities. Queen Máxima completed her look with matching leather accessoiries (a clutch from Namu Lux, gloves and shoes).
For this King's Day the Queen wore no precious bijoux on her dress. The Queen wore two diamond bracelets at the one hand, one with rubies and the other with sapphires. At the other hand the Queen wore a golden chain bracelet with various coloured gemstomes and a black leather-belted Cartier horloge (collection Tank by Louis Cartier).