Hiya, purple shampoo isn't anymore damaging than a regular shampoo except for the fact you have to shampoo all of you hair rather than just the roots, and for it to be effective you then wait for few minutes before rinsing.
I would say it depends how you like sulphate shampoos as doing this once every 3-4 washes will definitely brighten silvers and tone down brass.
But you could achieve the same effect by buying a temporary vegetable dye like manic panic virgin snow and applying that (some say to dilute with conditioner but virgin snow is so pale it needs to be used full strength really)
Or if you weren't keen on a full head of that, a tiny quarter teaspoon of a blue base purple dye mixed into a palmful of conditioner once every few Weeks works a treat.
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