Question: Why Does the Pool Turn Blonde Hair Green?
Answer: It isn't the chlorine that turns blonde hair green. Oxidized metals in the water bind to the protein in the hair shaft and deposit their colour. The metal that produces the green tint is copper, which is most commonly found in algicides, though it naturally occurs in some water. The bleach (Chlorine) that is added to a pool may be responsible for oxidizing the metal, but it's not the cause of the colour.
If your hair turns green, you can remove the discoloration by using a shampoo that chelates the metal. To some extent, you can prevent copper from binding to the hair by sealing the hair cuticle with a conditioner before swimming. Rinsing your hair immediately after leaving the pool will help protect it, too.