I was doing research to learn more about hair coloring and found this great post:
There is lots of great info on counteracting colors/color correcting and general color theory that helped me understand some things, and I recommend this as a good resource.
I now understand, for example, why when using an ash-based color on blond highlights, you can get a slight green tint (the blue in the ash mixes with the yellow/gold and makes green), and that to neutralize yellow/gold a violet base is needed, not ash base. Violet is opposite yellow on the color wheel so it neutralizes. Blue/ash would be for correcting orange tones but not gold tones.
This is definitely info worth reading so I thought I would share....