The flat saturated blue colour of Diana’s suit was a memorable element of the royal engagement photographs. It was a colour that was strong enough to become emblematic. This blue had acquired a new value, it was the blue associated with Diana. And so it became fashionable.
Kate Middleton has chosen a similar saturated deep blue dress (slightly darker) for the announcement of her engagement to Prince William. It matches the sapphire of her engagement ring, previously worn by the ever present Diana.