The reasons for the colourless fingernails of Duchess Kate and Queen Elizabeth II are a bit of a mystery. Why do the royals never wear coloured nails?

It appears to be due to the etiquette of the royals, because the members of the British Royal Family are actually prohibited from wearing coloured nail polish.

Why the royals don't wear coloured nail polish

Clear lacquer or delicate pink tones are allowed. In addition, the nails must not be too long and must always look well-groomed. Neutral fingernails are definitely practical: they go with any outfit!

The nail polish brand Essie is said to have revealed what the Queen has been wearing on her royal nails since 1989, according to OK! magazine.

"Ballet Slippers" by Essie is the Queen's favourite colour, which costs an affordable $7.95 (about £6). It's a neutral, pastel pink shade.

The Essie nail polish is the only one the Queen will wear. And Duchess Kate does the same. Among other things, on her wedding day with Prince William in 2011, she is said to have worn this colour.

For more on royal fashion, we also recently took a look at why the Duchess never wears the colour orange, and the one colour you'll never see the Queen wear.