When lightening your hair from brown to blonde, you are essentially stripping the pigment out of your hair. Because the undertones in your hair get darker as you age, even applying a peroxide lightener will not fully lighten the yellow or red undertones in hair, which results in a brassy color. To fix brassy colored hair and make the hair more white or ash blonde, apply a semipermanent toner dye to the hair to even out the color and remove any brassy tones.
Put on a protective gown to cover your shoulders or an old t-shirt you don’t mind ruining. Toners contain small amounts of peroxide with hair dye, so they will ruin any clothing they come in contact with.
Mix the toner as per the manufacturer’s instructions and apply it to the areas where it is needed most. The ends of the hair tend to lighten better, so the brassy tones usually appear near the scalp. Try not to apply the toner to areas that don’t need it.
Cover the saturated hair with a plastic bag, and let the toner process for the recommended time. Most products require 30 to 40 minutes of processing time.
Remove the cap, and shampoo and condition the hair to remove the toner. You will notice that the brassy color has disappeared.