Sparkling, bright and sharp ... and that’s just Mrs O’s fashion sense

In culinary terms, her mint green shantung silk skirt, sparkly silver mules and cute little sequin-speckled $298 (£207) cardie by US high street chain J. Crew were the equivalent of a granola fresh fruit plate: full of texture, with a dose of healthy colour on top. In contrast, Sarah Brown's ruffle-fronted dress by Britt Lintner, who is American born but London-based, was perfectly smart, but about as exciting as a bowl of corn flakes.

That Michelle Obama would pack a suitcase full of bright colours for her trip to London is no surprise. Rare is the woman who would seek to make citrus hues her trademark, since they make most women look like lemons rather than some divine, exotic fruit.

But since the success of her inauguration day look — a zingy dress and coat in a hue its designer, Isabel Toledo, described as "lemongrass" — Michelle Obama has continued to embrace those tricky shades of yellow, orange and lime that most women would rather jump off a tall building than wear.

On a recent cover of O magazine, she wore a bright orange dress and a short-sleeved yellow cardigan. The mustard yellow satin dress she arrived in last night proves just how adroitly she manipulates the cameras: it looked overwhelmingly upbeat on a gloomy sub-spring London evening.

In contrast with Mrs Obama's soaring colour confidence, Sarah Brown is always going to look rather mousy — but then, so would half the red carpet fodder in London. While we can expect Mrs Obama, during the rest of her trip, to fly the flag for young American designers such as Taiwanese Jason Wu, the Cuban Toledo and Thai-born Thakoon Panichgul, it is a dead cert that protocol will dictate Mrs Brown keeps the wilder reaches of Britain's fashion imagination strictly under wraps — high street wraps, that is.

While Mrs Brown's patronage would provide welcome publicity for talented young London designers, it is unlikely that this week will see her wearing anything more directional than Jaeger, Ben de Lisi or LK Bennett. Shades of Cherie Blair still loom large over Downing Street, with the Labour party undoubtedly keen to avoid any allegations that Mrs Brown is accepting "freebies" in the form of expensive designer clothes. Which rules out Alexander McQueen, then.

In any case, the PM's wife is not a fashion-forward dresser: she seems happiest in simple, knee-length dresses and suits in grey, deep purple or black. They might leave her wide open to allegations of dowdiness when compared with the vivid citrus splendour of Michelle Obama, but there are worse things than knowing what suits you, and more important things to worry about than suits.

Darned if I can think of many, though.