Hairstylist and colorist Jana Rago tells Real Simple that there are several reasons the tones in dyed hair can easily turn brassy. Some things are in your control, but some are not.
"These tones can turn due to sunlight, washing with the wrong shampoo that doesn't protect colored hair, or a mistake made by a colorist, like not using a toner," she says. "The sun will dry out the hair by opening the cuticle on the scalp, which will lift the color out, causing the hair to look brassy."
Colorist Megan Graham says that, once your hair is very brassy, you may need to make an appointment to see a professional colorist and specifically ask for a color correction. This involves several steps and isn't something you'd be able to do at home. You'll want to make sure that the colorist uses a toner that will work to keep warm undertones hidden.
Celebrity hairstylist Kristin Ess agrees and says it's always best to avoid color-correcting yourself (via Elle). "If you need any major colorwork, you should always go to a pro,” she says. "You can really mess up your color by going out of your 'hair color family' with any color product at home."
