Can someone explain hair toner,and it's purpose?Can it be used to remove brassy undertones in dark brown hair?How do you choose the right color?

A toner is basically hair color. When you bleach or lighten your hair, it leaves it in a brassy raw state, so you would need to apply a toner to neutralize the yellow/orange tones. If your hair is dark and has red/orange undertones, try using an ash base hair color to neutralize the unwanted tones. Choose a dark ash brown hair color. Leave the color mixture on your hair for tens minutes then rinse it out.

Because it helped me to understand what a toner will do to your hair.