Most people you ask will tell you that it is the chlorine in swimming pool and hot tub water that causes hair to turn green, but that is actually a myth. Chlorine in pool and spa water causes metals such as copper, iron, and manganese to oxidize. Think of what happens to copper used in pennies, pots and pans, and architecture over time, the process is the same. After exposure to elements and over time the copper turns from its original color into a greenish turquoise as the minerals oxidize. Repeated exposure to the oxidized metals floating in chlorinated water can often cause light colored hair to appear to have a greenish tint. So contrary to popular belief it is actually metal in the water turning hair green, not chlorine.
Now, what to do about it?
Using a metal removing compound that is compatible with your sanitizer like ProTeam Metal Magic, can help remove the metals from your spa water and into your filter where they can easily be removed with a filter cleaning.
Controlling the metals in your water will also help out with stain and scale build up on your spa walls and in pumps and heaters.