Let’s be honest about blonde hair for a sec. Apply your purple shampoo generously to parted, dry hair. Can I leave purple shampoo on for an hour? There is an age old fix. Remember, leaving purple shampoo in your hair for this long can result in some lilac staining. Too many have no idea how to use it right. What’s more far fewer know of a hack that makes it even more effective. "Don’t apply it on dry because the hair is more porous and it will grab too much of the purple tone," cautions Kandasamy. But there’s a problem. It is most effective once or twice a week during your normal bathing, and leaving it on for 5-10 minutes depending on the level of brassiness in your hair. That’s to use purple shampoo to cancel out unwanted brassiness. Keeping it looking pretty, super bright and free from brass is not easy. This differs by brand so always check the instructions: some suggest leaving the product in your hair for as long as 10 minutes. How long should I leave purple shampoo in for? To use a purple shampoo, wet the hair and lather on the formula. In this video, YouTuber Kristina Angelina, leaves the purple shampoo in her hair for 30 minutes, for an intensive treatment to take her silver hair from blonde-ish to bright silver.