What will it do to my dirty blond hair?

Purple shampoo is meant to cancel out yellow tones in blonde and gray hair. If your hair has orange tones you will need to use blue shampoo. If your hair has red tones you will need to use a green shampoo. This is based off of color theory and the color wheel. Colors that are opposite on the color wheel cancel each other out. (Purple is opposite of yellow, blue is opposite of orange, etc.)