I've been using sulfate free shampoo for over a decade and can't remember any of them making my hair really dry. I've had more problems with some of them being a bit too heavy for my fine hair. There are so many choices nowadays for sulfate free shampoo and there are a lot of good ones.
Some purple shampoos can dry your hair out, especially some of the cheaper ones. I use Pureology's Strength Cure Blonde Shampoo once every week or two and it helps keep my highlights from getting brassy and it's not drying at all. It's pricey but it's one of my favorites and it lasts forever. You wouldn't want to use a purple shampoo every time, regardless of how gentle it is.
Another product that I like is IGK's Mixed Feelings Purple Leave-in Blonde Toning Drops. You add as many drops as you want to shampoo, conditioner or leave in products. It works pretty well and it saves you from having another bottle of shampoo in the shower.