Just when you thought stripes were well and truly over, they've been re-invented in another guise - in dégradé. Dégradé, where colours are gradually shaded instead of clearly demarcated, is the modern, minimalist version of tie-dye, though few designers would flag up that association. A Valentino dress is out of reach for most of us, so Jess Cartner-Morley picks out some less fiscally challenging examples of the look