They can even be used as a way to 'try out' a new shade. Just like a box dye, which you otherwise have to mix yourself, the longer they’re left on, the stronger the result, meaning they can be anything from a quick refresh to a temporary colour change. More of an icy girl? No problem. Anna Short, senior colour director at Daniel Galvin said her go-to for Nordic-haired clients is Kérastase Blond Absolu Masque Ultra Violet Treatment, £35 . "It really prolongs the life of those really crisp, almost grey blondes," she explained. "I advise clients to use it once a week and then they don’t need to come in for toning treatments as much. The purple pigment takes away any brassiness that might have accrued and really brightens the hair without actually shifting the colour. It’s really common to get fade or unwanted orange tones with blondes, just like how wearing a white T-shirt around London all day will mean you come home looking a little bit grey and grimy. The pollution and the mineral build-up can really dull blonde hair."