I've heard tomato ketchup works, as the red is supposed to neutralise the green!
I found this:
Green hair can definitely be a problem and will tend to impact blondes the worst. Chemically created blondes tend to have hair that may be more porous and allow chemicals to soak in easier.
There are a lot of reasons why hair will turn green. A common culprit is chemicals in any type of water that the hair is exposed to. This water can be from swimming in water with chlorine or it can be from chemicals that are added to drinking water.
Well water may also cause some hair to take on a green cast.
How do you remove the green?
If you've been swimming in chlorine check out your nearest beauty supply store for products designed to remove chlorine from your hair. You can also try clarifying shampoos. Philou's Green Apple will often help remove chlorine or other chemicals from the hair.
If that doesn't work you can try neutralizing your hair with an application of either tomato sauce or ketchup. Apply it to your hair and carefully massage it in. Wrap your hair up for a few minutes with an old towel. Then shampoo and apply a good rinse out conditioner.
If you still have green hair, head for the nearest hair color expert and have them work their magic on you.
Prevention is the always the best medicine for hair problems. Once you get the green out, take care to avoid using any hair care products that contain metallic salts. Also, wet your hair before swimming to slow down chlorine absorption. Once you get out of the pool, rinse your hair with Club soda. Chlorine is a copper-rich bleach and the Club soda will neutralize it and wash it out of your hair before it can turn green.
If the problem is with well water, shampoo your hair in distilled water. Buy the cheap jugs at the nearest store and use it to shampoo and rinse.
Tomato sauce or ketchup works to neutralize the green. It should also work on hair that is green from well water.