Mourning clothes followed a strict hierarchy. The closer the relative, the longer and more 'deep' the mourning period. Mourning was relaxed after six months when mourning for a more distant relative, such as a grandparent, in Alexandra's case in 1901. After six months, you could stop wearing black and move into greys and mauves. Queen Victoria died in January, and Anastasia was born six months later, in June. Therefore, Alexandra would have been out of 'deep' mourning, which necessitated only wearing black, by the time this photograph was taken.
Remember, that photograph is coloured and probably does not reflect the true colour of the dress. In the actual uncoloured photograph it looks closer to a grey or possibly mauve dress, which would fit the lesser mourning period Alexandra would have moved into by the time Anastasia was born.
Rachel
xx
