If you want to really understand how this works, you can try mixing some yellow and purple paint together … Mix half shampoo and half color without a developer. Just like purple shampoo, blue shampoo is formulated to bust brassiness in color-treated hair. If you’re brunette and you want to maintain cool, brass-free hair color, reach for blue shampoo. How Often Should I Use Blue Shampoo? The purple shampoo bottle will tell you to apply it to wet hair in the shower for 10 minutes, but is to apply it to dry hair and leave it on for upwards of 15 minutes and THEN wash it out. We only mix shampoo and hair color to refresh your previously colored hair. Hairstyling pros recommend sudsing up with blue shampoo … Specifically, blue shampoo … The only difference is, where purple shampoo is intended to be used by blondes, blue shampoo is better suited to brassy brunettes. If you’re blonde, opt for purple shampoo. (Color Only) Wet your hair, mix the 2, and … It is very simple. What purple shampoo achieves is that it cools down the warm tone of the yellow shade in your hair and makes it more platinum-like or greyish. Bottom line?