You should let the purple shampoo sit in your dry hair … For the best results, we advise potential buyers to choose a shampoo with a deeper, more saturated purple … Strength Cure Blonde Shampoo even beefs up strands that have been weakened by the hair color process. Light purple shampoos don’t usually have enough pigment to make much difference. I dye my hair black but like pitch black, but after awhile it starts lightening up so my friend said if you use purple shampoo it helps keep it dark. I'm wondering if it's true because I know purple shampoo is for blondes to remove brassiness so I'm just wondering if it "really" does help make your hair … Yes, this purple shampoo on dry hair has thirst-quenching properties as well. Designed for natural and highlighted blonde or silver hair, this purple shampoo neutralizes brass and copper to leave hair brighter in just one wash. The darker the shampoo is, the more effective it will be in countering the yellow tones in your hair. For example, if your hair needs an intensive color correction, especially if you have very light hair, like platinum, you should apply the shampoo as a toner on dry, divided hair. It contains … My friend told me that purple shampoo helps darken your black hair color.