Toner is a dye that cancels out either warm or cool tones in the hair so that the hair is neutral. It is a deposit only color. Toning is especially important when someone wants white hair. Bleaching removes pigment in the hair strand but leaves behind undertones. Some are warm, like orange, others are cool. Depending on the left over tone, you select the opposite shade of toner to cancel it out. This leaves you with white hair.