Contrary to popular belief, the greening of hair from spending a lot of time in swimming pools is not the direct cause of chlorine. Your hair turns green from the presence of hard metals (copper, iron, and manganese, in particular) in the pool water. Think about the color that old pennies turn after time or the Statue of Liberty. The chlorine in the water speeds up the oxidation process and the metals are able to bond to your hair, causing the greenish hue. While this happens to all types of hair colors, it is most visible in the people with blonde or lighter hair.

Green hair is usually caused by copper being dissolved in the water and oxidizing due to the change in pH by the chemicals that keep the water clean and the amount of copper in the water source. It precipitates out and stains the hair green (think of what color copper is when it oxidizes).