Hair Color and Dish Soap

Dish soap
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Q: My hair is naturally a dirty/ash blonde. I usually just get partial highlights, but my hairdresser recommended doing the underneath portion of my hair a little darker, it ended up a chestnut brown, which I do not like.
She told me to try using Palmolive dish soap to lighten it up. Does this actually work or do you have any advice? I would even be willing to use a box color to lighten it if it wouldn't turn my hair orange.

A: The principle your hairdresser is using in advising this is sound, but with some hair color formulas may be less effective than with others.
Most dish soaps (and other cleaning soaps for that matter) are highly alkaline and can swell the cuticle of the hair when the hair is exposed to them. Since hair colors typically deposit color into the hair by raising the cuticle layer, using cleanser to raise the cuticle and allow some of the color to wash out is a reasonable theory.
It works along the same principle as the "color additive shampoos" designed to be used to keep your hair color fresher longer after a color service. They deposit small amounts of color with each shampoo to help prevent your permanent hair color from fading.
In the case of using an alkaline cleanser on color-treated hair, you are trying to remove small amounts of the color from the hair without having to resort to bleaching or another high-lift color process.
Be sure to use a good conditioner after shampooing with any dish soap. The alkalinity will cause the soap to act as a hyper-clarifying shampoo and can easily strip the hair of all its protective oils, etc. It may also be a good idea to follow up any rinse-through conditioner with a leave-in conditioner spray as well as a smoothing serum to help keep the cuticle layer flattened.
See also:
Hair color fixes
Hair color and coloring
Stripping hair color
Shampoo, conditioner and hair color fading