Complementary colors are used to neutralize unwanted tones when mixed together as they cancel out one another.

In your case a green base color will neutralize any unwanted red tones in your hair. The green will cancel out all red tones in your hair.

no you need matt, metallic or green tones

If you do color you use math by using the scale to measure the grams of color, tones and developer you need.

Any haircolor that says Neutral blond . This way you will not get any yellow or red tones.

Normally you are suppose to mix the hair color booster in with the mixture while you are lightening your hair.

Hair color boosters are used enhance the performance of the color and to avoid orange and gold tones and does not change the color of your hair.

The boosters help achieve the extra lift that you would want in your hair.

However once you get your hair colored again, you can add the booster then.