Since the Mistress of the Robes just behind the Queen and the other lady between Edward VII and Haakon VII are also both in black, the Court must have still been in mourning for Alexandra's father, Christian IX of Denmark, who had died earlier that year.
I don't know the source, but I have read on another board that Alexandra never emerged from half mourning following Prince Albert Victor's death. Looking at these details from paintings below we can see that prior to the prince's death Alexandra was portrayed wearing red, gold, blue, and yellow.
Yet after her son's death, she is only seen in white or very pale colors.
Some sources list white as an acceptable color for half mourning, so I suppose Alexandra could have loosely followed the guidelines although most colored stones would not have been acceptable. The most notable exception would have been the Coronation, with the golden gown, emeralds, and possibly other gemstones. In most of the photos from her later life she appears to be wearing black.