There are plenty of purple shampoos out there, but finding one that's sulfate-free is actually really, really tough. Not to worry, though, because we've done the work for you and rounded up the best sulfate-free purple shampoos just ahead. They all work to neutralize brassy undertones in blonde (or gray, or silver) hair, and despite being sulfate-free, they still lather up into a nice, satisfying foam. Note that brunettes can use purple shampoos to neutralize orange undertones, too.
You can alternate using your purple shampoo with your regular shampoo until you achieve your desired shade of blonde/white/silver/gray/brown, and after that, go down to once a week to maintain that perfect color. Remember, the longer you leave your purple shampoo on, the more effective it will be — but don't leave it on for too long, lest you're aiming for lavender hair. About two or three minutes should do the trick.