According to PureWow, dry hair may absorb more of the purple pigment, but you'll get an uneven color. That's because drier hair will absorb more product, so the end result will be a little "patchy." The best way to get an even tone is to use the purple shampoo on wet hair. If you find that you have some dry areas, like the ends of your hair, you might want to add some conditioner as well to moisten it up a bit. 

However, if you don't have blond or highlighted hair, Stylist says there's really no use in using purple shampoo at all. The reason some TikTokers who use it come out with platinum blond hair is because they already had blond hair to begin with. The purple hue shampoo was made to tone down the yellow in blond hair to make it more silvery blond. If you've got dark hair, you'll most like just be wasting a bottle of purple shampoo when pouring it on your head.