Susannah Frankel: Ready To Wear

What a big – or should that be little? – show-off Natalie Portman is. Not only is she wearing the dress of the summer season – it's by Lanvin and every high street name worth mentioning has by now reinvented its ruffled loveliness – but also it is a bright, even violent purple. What's so impressive about that? The fact that this is an extremely difficult colour to wear. Any fashion follower worth her credentials loves a challenge, after all, and purple is the mother of all challenges.

Not just any old purple will do – that would be too simple by far. Lilac tends to bring to mind your granny at a wedding, for example – Giles Deacon likes it this summer though where the shade rubs shoulders with sugared almond pink for some of the most unashamedly girlish designs imaginable. Anything approaching plum – the sort of purple that is associated with Biba in its heyday – is rather too retro for comfort, although draped jersey dresses in this same hue at Louis Vuitton and mixed with accessories in other, more show-stopping purple shades might suggest otherwise.

The dominant mood decrees, though, that the brighter and bolder the purple the better. At Comme des Garçons the shade of the summer is a distinctly un-summery ultraviolet, a vivid purple worn with pink ankle socks and equally virulent sandals to match. At Alexander McQueen a kimono-sleeved gown comes in fuchsia pink with an equally attention-seeking purple skirt and the effect is dazzling in a vintage Yves Saint Laurent kind of way.

If purple is big this summer, it's even more so for autumn when, far from having tired of the colour of the moment in the manner to which they are accustomed, designers have used more of it still. At Dior, Mrs Robinson has been given a colour-drenched makeover in the form of Sixties-line dresses with wide-brimmed purple hat and high-heeled sling-back peep-toed sandals to match. Ann Demeulemeester has traded in her monochrome palette for Mongolian lamb the colour of Highland heather. Donatella Versace is enamoured with papal purple.

In the end this is purple with courage in its conviction, the ultimate colour for big entrance dressing and best suited to one brilliant and brave statement rather than an overcomplicated layered effect. Those unconvinced by the power of head-to-toe purple might like to try a pair of purple tights for size. They looked reasonably manageable under a black skirt on Sonia Rykiel's catwalk and quite impossible teamed with a bright orange tea dress at John Galliano. No prizes for guessing which catches on.