Greenish Hue Hair Color
A: There are a couple of explanations as I see it. There is the greatest likelihood that your hair is showing greenish hues because the natural color is graying and the color that you are using has a greenish base color. This is commonly the case with many neutral browns.
Whatever the cause, however, what you need to counteract the results is a mild color correction. My best recommendation is to visit your local beauty supply store and pick up a level light hair color (level 9) that has a red base color and a 10-volume peroxide developer.
You also need to get a mixing bottle, gloves and a plastic cap. Combine one ounce of the hair color (1/2 the bottle) with one ounce of developer and two ounces of your favorite conditioner. Apply the color mixture to your dry hair, cover the hair with the plastic cap and leave on for 10-15 minutes.
This should neutralize the green tinge of your hair. If your hair still shows green, you can repeat the process. It is important that you use a light hair color because the color is deposit-only and will "stack" on your current color. To use a color that is darker (such as one that is the same as the current color you are using) will make your hair much darker than you may expect.
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See also:
How to color hair
The hair color levels
Why and how hair turns gray
Hair with a green cast after doing foil highlights