When two of the world's most photographed women step out in the same colour, it's clear something's afoot. But while "most photographed" might not be the same as "most fashionable", where the Duchess of Cambridge and the First Lady lead, shoppers will follow.
The colour in question is teal – well, sort of. Varieties of blue-green shades from petrol to peacock have cropped up in front of the cameras over the past week, draped around all sorts of high-profile shoulders.
The Duchess of Cornwall was spotted at Cheltenham wearing a velvet-trimmed wool coat in the colour, while Kate wore a teal dress and jacket from LK Bennett to accompany the Queen at the start of her Diamond Jubilee tour. Michelle Obama's elegant Marchesa gown, worn at the White House gala on Wednesday night in honour of David Cameron's visit, plundered the darker end of the teal spectrum, and the Emilia Wickstead funnel-neck dress worn by Samantha Cameron on her visit to the 9/11 memorial in New York highlighted the shade yet again.
They were all pipped to the post two years ago, of course, by Sarah Brown, who kicked off the first-lady trend for teal when she donned a pintucked Osman Yousefzada dress to take her leave of No 10 in 2010.
The appeal of teal is obvious: classic and refined, it gives the impression of colour without the wearer suffering the gravitas-wilting pastels or neons favoured by pop stars and children's TV presenters. Its depth is just as flattering and subtle as navy or black, but far less severe. For any woman working in an official capacity, blue is risky: too dark and you look like a policewoman; too bright and you enter Margaret Thatcher territory.
There's teal on the catwalks this season at Dries Van Noten, Proenza Schouler and Marc Jacobs – but the colour rose to wider prominence six years ago when the first copies of Roland Mouret's figure-hugging Galaxy dress started to appear on every high street.
But there's also something nostalgic about teal, which was first recorded as a shade in 1917 – Downton Abbey days. If these public figures have co-opted the shade, perhaps – just perhaps – it's a sartorial nod to austerity, or maybe a return to the era of noblesse oblige. The latter might not go down too well with Michelle Obama – but her dress is petrol blue, anyway.