Purple shampoo and a toner treatment are the best ways to fix brassy highlights. "Brassy" refers to the warm reddish or orange tones in blonde hair and is in contrast to "ashy" tones, which are those with bluish hues.
The most common and most recommended course of action when attempting to fix brassy hair color is using purple shampoo and conditioner. They have purple pigments, and because purple is the opposite of orange on the color wheel, they can cancel out warm orange color. It is recommended that these be alternated with color-protecting shampoo and conditioner made specifically for dyed hair to keep the hair healthy.
Another option for color correction is a toner treatment. Toners are a type of gloss and are applied at the end of a normal dye job to harmonize and even out the tones. A purple-hued toner works much like purple shampoo, except that it is more potent. These are typically done in a salon, but there are also a number of products for do-it-yourself home application as well.
Some hair experts also recommend purchasing a shower head filter to keep hair from getting brassy. Iron deposits in shower water can make hair take on reddish hues in time.