Green hair can be caused by various external sources
i) The deposition of pigments from copper plumbing.
ii) Calcium Hypochlorite, Sodium Hypochlorite or other bleaching chemicals and chlorine used in swimming pools etc.
iii) Other possible sources of contamination: tar shampoos, skin drugs containing mercuric oxide.
A perfectly formed virgin hairshaft with cuticle intact is usually impervious to any of the above. However hairshafts become vulnerable following damage associated with over exposure to ultra-violet light, over zealous blowdrying and chemical (oxidative) bleaching, dying, permanent-waving or straightening procedures.
Blonde hair-shafts are more likely to show green pigments.
I know of no safe treatment to remove such colour, but expect the colour to fade - slowly.
This type of contamination does not usually cause hair damage.