Brassiness is common in blonde color-treated hair. It occurs with oxidation, often from sun exposure. Your water may also be to blame if it contains high levels iron. Using warmer, less golden colors can prevent brassiness, but all colors eventually oxidize either because the center of the cuticle is starting to show or the color wasn’t lifted enough.
Brassy hair brings out orange undertones in the skin, which can make you look sallow. Because it comes from the naturally warm undertones in your hair, it may cause a clashing effect between your hair and skin tone.
One way to fight brassiness is to wash with a blue- or purple-tone shampoo, like Emmett Cooper Support Wash-but you’ll need to use it consistently. Your colorist can also apply a toner with a violet base to deposit some counteracting color.
Another option is simply darkening hair a shade or two, which can balance out any brassiness.