When I am an old woman I shall wear purple," begins Jenny Joseph's well-known and, to my mind, faintly depressing poem, Warning. "But maybe I ought to practise a little now?" it concludes. "So people who know me are not too shocked and surprised/When suddenly I am old, and start to wear purple."

Perhaps these lines were running through Madonna's mind when she got dressed earlier this week. Unless she was musing on the emperors of ancient Rome. Anyway, she appeared empurpled from top to toe: a purple long cardigan over a purple camisole, accessorised with purple ankle booties, a purple sequined belt, a pendant with a purple M on it, and purple glasses. Setting off off all the purple (and perhaps answering for the "red hat which doesn't go", in Joseph's poem), was a shocking pink bag. And, to emphasise that the boots were of the correct, ankle-length variety, she wore a pair of abbreviated jeans.

Thus, in just one outfit, Madonna adroitly combined at least three of the looks that have been designated as key this autumn: truncated boots, purple, cardigan. And maybe the sequined belt: I'm not sure. As always, her outfit was instructive. Is it possible to wear too much purple? Yes.

We are indebted to her. These days, when not working as an international rock star, Madonna provides a no less valuable service as a fashion-tester for women of around her age: she wears the clothes so we don't have to. In previous seasons she has taught us how much brown tweed a fortysomething person can reasonably wear (skirt, or jacket, but not both at the same time), and often, as this week, demonstrated the dangers of taking coordination too far. Remember the rose-covered shoes that matched the rose-covered dress? On this occasion, she shows that a person hoping to indicate vestigial fashion awareness through the adoption of a passing trend should perhaps stick to one thing at a time. Two, max. Especially if that thing is purple.