Foil Highlights & Green Cast

Highlighting product for hair
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Q: I did foil highlights on my daughter's hair using 30-vol and it had a green cast. Why is this?
A: There could be a number of causes for the greenish cast - ranging from heavy metal deposits in your ground water, to interaction with previous colors applied to the hair. It is also possible that if your daughter's hair is naturally dark and/or ash-toned that the greenish cast is a result of that.
The strength of the peroxide you used has little bearing on the greenish result. The difference a stronger or weaker solution would make is to shorten or lengthen the amount of time needed to lighten the hair. This greenish cast is purely a result of the reaction of an existing factor in the hair to the lightening process.
The solution to the problem is to use a toning color to counter the greenish tint. Select a hair color that is similar in shade to the color of the highlights and has a red color base. Mix the color using 10-volume peroxide and conditioner in equal parts (i.e. 1 oz. color with 1 oz. 10-volume peroxide developer and 1 oz. of your favorite rinse-through conditioner). Mix these ingredients well and apply to the hair. Leave the mixture in place for 15-20 minutes.
This should counter the green and leave the hair a more natural color.
See also:
How to color hair
Pools and green hair
Hair problems caused by hard water