'Don't wash a Prada bag with Windolene,' advises New York super colourist Brad John. 'Use shampoo and conditioner formulated for colour-treated hair - they won't strip colour from the hair shaft and can keep brown from turning brassy or orange, and blondes from fading to ashy tones. And wait as long as you can bear between washes. No matter how gentle the shampoo, over-washing is deadly for colour.'