She had worn no hat when the custom is for royal women to wear hats on such occasions, especially when accompanying the Queen; 2) She had worn black and white, which are two of the royal mourning colours, along with mauve; 3) She had worn a French designer, not a British, which was in breach of the protocol whereby British royals wear British designers to drum up support for British trade when on royal duties. While it had been just about acceptable for her to wear a wedding dress from the French couturier because its designer was British, the same did not hold true for everyday attire.
