how do i know if i should use a green, blue or violet base toner to correct a brassy red orange color mistake

Well, if you have red tones in your hair and want to remove them, you will need to use a green base hair color to counter-act the red tones. But if your hair is more orange than red, you must use a blue base color to neutralize the orange hue in your hair. You will find these types of hair color at a beauty supply store.