Cover Gray Hair with Brown

Hair color application
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Q: I have a hair color question. I have been dyeing my hair since I was 17 and now I am 49. My hair grows so fast that I have to color it every 3 weeks. I dye it medium brown, which looks dark brown.
I think it is too dark, but I feel light brown doesn't cover the gray. Any suggestions cause I really would like to color it a light brown.

A: One of the greatest difficulties I face as a hairdresser is adequately explaining to people how hair color works.
In a nutshell, when you are dealing with a permanent hair color formula being applied to hair that is a mixture of gray, naturally colored, and previously colored hair, you are adding pigments to not only the gray portions of the hair, but also to the naturally pigmented and previously colored portions of the hair as well. This addition of pigment can result in a much darker result than you expect since you are darkening even the previously darkened hair.
This is why permanent color is usually applied only to the new growth of hair during the first half of a retouch service then is added to the rest of the hair for the last 5-10 minutes of the color service to freshen up the color. It is also why the aim in salons when covering gray hair with color is to blend away the gray in an attempt to create a natural-looking color.
Depending on the amount of gray hair present, the color used is generally a few shades lighter than the naturally pigmented hair and the result is that the gray hair becomes a highlight color. The alternative would be to use a shade equal to the naturally pigmented hair which would create a flat color result.
Addressing your problem specifically:
It is important to note that we will have to deal with the previously colored hair as a factor in considering what to do to get your hair the color you want. The problem I have is that I cannot make any specific color suggestions since I don't know what color was previously used or what your natural color is, and therefore what may be in conflict with those colors.
In the most general terms the fact that you want to go from a darker brown to a lighter brown means that you will have to lighten the existing color. The exact process needed will depend on how much of a lightening effect you want.
Given the unknown factors involved and the chance for errors and potential damage to the hair I STRONGLY encourage you to seek the assistance of a professional stylist in order to make this transition. You can specify to him or her that you need something you can maintain yourself if that is a major consideration, but I do not recommend trying to do this kind of color shift yourself.
See also:
Gray hair Q&A
How hair color works
Hair pigment production