Yes, thank you for bringing the white-mourning question to this specialized thread about mourning customs.
Like Eddie told about white-mourning being a custom of the medieval queen was completely unknown to me. I was also not aware that the Queen Mothers ''white-wardrobe'' was originated from this same custom. I knew it was an alternative way of mourning for her mother, but I never knew the idea behind it. I just thought the dressmakers and Queen Elizabeth had to find a quick alternative for her dresses because Elizabeth's mother had suddenly died and black was not the right representation in the difficult times before the war. Or so I read in the information of the Royal Collection:
''At a time of political turmoil, the State Visit was intended to reinvigorate the entente cordiale and to reinforce Anglo-French solidarity against Hitler's Germany. Five days before the date of departure for Paris, Queen Elizabeth's mother, the Countess of Strathmore, died and the visit was postponed by three weeks until 19-22 July. Hartnell had to remake the Queen's wardrobe in its entirety, substituting 'many lovely colourings' with something more appropriate to the period of Family Mourning. Black was not a practical choice for the height of summer and seemed inappropriate for the mood of the time. The couturier's last-minute suggestion that white might be a suitable colour met with the Queen's approval. Accompanying the King, Queen Elizabeth departed from Buckingham Palace in black and stepped from the Royal Train in Paris dressed in white. ''
Other royal women that I know of were Queen Olga of Greece and Queen Wilhelmina of the Netherlands together with her daughter Juliana. But I think Wilhelmina's ''white mourning wardrobe'' can not be traced back to the medieval queens. At least I never read such a thing. Aldo Wilhelmina and Henry did not have a good marriage. She and her husband had promised each other that they both would chose to have a white funeral and wear white mourning, because the both did not see death as an the ultimate ending of their life. But as some sort of new beginning or death as the way to a new life. So it had to do with faith. Henry's funeral was complete in white and Queen Wilhelmina and her daughter wear white mourning outfits during the mourning period. Just three months and a few days earlier Queen Wilhelmina's mother Queen Mother Emma had died and the mourning custom had been black. When Queen Wilhelmina died in 1962 her funeral was just as white as her husband had been. Her daughter and granddaughters followed the coffin in their white outfits. Juliana must have been impressed by this and when her own time came to go (2004) - her funeral was in purple with white accents. And again her four daughters appeared in white mourning.
Queen Sophie of Greece also weares some different mourning outfits during special celebrations and like Eric stated princess Nicholas did the same.
Queen Wilhelmina and her daughter Crown-Princess Juliana of the Netherlands in white mourning when visiting the grave of King Albert of Belgium in the church of Leaken (1935).
Queen Wilhelmina visiting a exhibition in the Vondelparck in Amsterdam (1935).