People believe that it's the chlorine in a pool that turns hair green. That's not true. Fair hair turns green in a pool for the same reason the Statue of Liberty turns green: copper.
Long summer months in the pool often have an unpleasant side effect on fair-haired people. After a certain number of long dips in the pool, they acquire a green tinge to their hair. Most often this is attributed to the chlorine in the pool water, but that's not the case. Chlorine doesn't turn hair green. Nothing turns hair green - except green hair dye.
It's not the hair that turns green in the pool, it's metal. Specifically, it's copper. Over time, and with exposure to oxygen, copper oxidizes, transforming from a shiny yellow-brown to a dull green. Copper is already suspended in water, especially hose and well water. One or two parts per million of oxidized copper can give hair a green tint. Once copper is oxidized, it tends to bind to certain proteins. Some of those proteins are found in hair. (Other exposed body tissues are sloughed off too consistently to make the green tint a problem.) Summer months make the green tint more noticeable, since the sun bleaches hair. To be fair, chlorine does speed the oxidation process, but a more common culprit is the algaecides that people add to their pool to keep it clear. These contain copper and make the greening process more intense.
There are special shampoos that strip the oxidized copper off of hair, and people can prevent the copper from latching on at all by covering their hair in conditioner. Alternately they can take pride in the fact that their hair is copper-plated. It works for the Statue of Liberty.