There's a wedding dress that wasn't white - and it triggered one of the greatest crises in the history of the British monarchy. When Wallis Simpson married the former king Edward VIII on 3 June 1937, she wore a dress in an unusual grey-blue colour, deliberately chosen to match the colour of her eyes. Today that shade is called "Wallis blue".

Their love scandalised Britain. Simpson was American, twice divorced - unacceptable to the royal family, the government and the church. Edward gave her a Cartier engagement ring with an emerald of nearly 20 carats, and then on 10 December 1936 he abdicated the throne so he could marry her, becoming the Duke of Windsor. When a king chooses a woman over a crown, the whole empire is left arguing over who actually lost more.

The dress was the work of the Paris house Mainbocher - silk crepe with a body-hugging cut and a short jacket with a ruched top, buttons down the front and gathered sleeves. A high collar set off with a sapphire-and-diamond brooch, a straw hat by Caroline Reboux with tulle and feathers, and jewellery from Van Cleef & Arpels and Cartier. Everything was chosen to send one message - this woman writes her own rules.

Not a single member of the royal family attended the wedding. The dress is kept today in the Metropolitan Museum in New York, but it has faded to cream - the original "Wallis blue" can practically no longer be recreated. There's something symbolic in that: the colour that caused an uproar faded with time, but the story of the woman who shook the foundations of the monarchy for love - that stayed alive.