Oxidized copper, not chlorine is what turns blonde hair green.  Oxidized copper can be high because of increased copper content in local water or because of pool water.  To prevent your hair from turning green, you can decrease exposure by decreasing time in the pool or protect it with a swim cap.  You can increase the protection by applying a conditioner the hair prior to applying the cap.  If you choose not to wear a swim cap you can decrease some risk by applying the conditioner prior to getting into the pool or rinsing the hair after each “dip” in the pool.  There are over the counter shampoos that contain ethylenediamene tetracetic acid (EDTA), these shampoos work to decrease the hair discoloration.