Brassy Hair Color

Hair coloring
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Q: I recently colored my wife's medium brown hair to a blonde, but it came out with a brassy color to it. We put on a semi-permanent medium dark brown over the top of it, but the brassiness has turned somewhat red. I know this color will wash out soon, but she has decided to permanently dye her hair to a fairly dark brown.
My question is: Do we need to neutralize the brassy color before dyeing it darker by applying a toner, or can we simply dye over all of it with a darker color? If so, do we need to pick a dark brown that complements her cool skin tone? Please help.

A: Okay in cases where you need to make sure that a hair color error (brassiness, overtones of one color - in this case, red) needs to be dealt with. You can either address the issue separately or make choices based on the color goal you have.
With what you describe in the red cast to the current color result, and the goal of going to a dark brown color ultimately, I would suggest that you look for a color that uses an "ash" base. This is a cool color base and will suit what you say is her skin tone, and will help to neutralize the redness of the current overtones in the hair.
Depending on the precise shade of the hair's current color, and the color you want to go to, you may want to consider using a color that is a little lighter than your target color, since you should be using a deposit-only process, the lightness levels will be cumulative, so that adding a level 3 color to hair that is level 6 may end up darker than expected. If the hair is much lighter – perhaps level 8 or above – you should be fine.
Please bear in mind that I am referring to professional color formulas and NOT box hair color kits from grocery stores or pharmacy stores. In order to use box color kits you need to read the boxes carefully for any potential problems and conflicts with any other types of hair color.
See also:
How to color hair
Cool and warm hair colors
Hair colors and our color palette