It's hidden behind the Sunday Times paywall and buried in the article, but Tim Shipman's profile of Boris Johnson contains this bombshell:
I think back to the photoshoot, when Boris runs his hand through that bird's nest of platinum hair. "This is the real thing," says Boris. "It's all natural." But you do die it, don't you Boris? I say. "Yes," he admits. Real but enhanced, a little like the public personality.I had hoped we would learn one day that it is a wig, but I will settle for this as a reminder of how carefully managed Johnson's unmanaged image is.