Blonde hair goes brassy when bleaching or lifting does not remove all of the underlying pigments in the hair. These underlying pigments that remain after coloring/lightening are yellow, orange, and red in color. As the cool pigments (toners or hair color) fade, the warm underlying pigments left behind after coloring become visible, causing a brassy appearance. Using a purple shampoo will deposit cool tones back in hair, counteracting the warm brassy tones, leaving the hair looking cooler and brighter.