Many of the relatively few biographies of Queen Alexandra emphasise that, after the death of the Duke of Clarence, she continued with half mourning for the rest of her life, favouring muted tones of mauve, heliotrope, lavender, violet etc.
Kate Strasdin,
Inside the royal wardrobe. A dress history of Queen Alexandra, 2017 endorses this view, and it is certainly very common to see press reports of such colours being worn for day and evening wear during her tenure as Queen.
Yet, Queen Alexandra did not inevitably use these colours as queen.
I haven't ever attempted a full scale listing of the all the colours of her Court gowns or day wear reported in the press, but I think it is important to recognise that she did use colour and, at times, vivid colour.
For the State visit to Berlin in 1909 she was reported to have worn a dark turquoise gown (State banquet), one was described as midnight blue (Opera gala) and for the State Ball she wore a yellow gown with golden sequins.
For the wedding of Princess Margaret of Connaught in 1904 her gown was described as being electric blue draped with a fine black net and heavily embroidered.
Below are a few other descriptions of the Queen's dress colours. Most press reports from the Court functions give a dress colour but I haven't waded through what I have.
1902.
1904
1904
1909
1908 French State visit -- a gold gown
1903 blue for wedding of Princess Alice of Battenberg
I haven't yet ever hunted for a description of Queen Alexandra's jewels on this occasion, but the very blurry photo below suggests to me that she wore the tiara we associate with the amethysts, but I suspect that on this occasion she may have had the sapphires inserted.
I find this 1908 press comment on the Queen's choice of colours very interesting. It also seems to confirm that Queen Alexandra's choice of black did not always indicate mourning.