She wore this outfit on a previous occassion, when she was visited by the japanese prime minister at Huis ten Bosch I think.
Funny to see her (and other royals) attending functions at the peace palace, considering her grandmother had to be persuaded to open it. I believe it had a catholic statue somewhere inside. She also disliked the idea of a peacepalace (or conferences) on dutch soil as she 1) found it humiliating as it showed the irrelevance of The Netherlands and 2) she did not believe in these conferences all together, just lies (she was right, sadly).
Likewise Wilhelmina refused to open the famous Rijsmuseum btw, as some windows looked to catholic.