Taking aspirin with water won't help, but rinsing your hair with aspirin dissolved in water will, according to Manhattan dermatologist Amy Wechsler. It's actually the copper in chemically treated pools, not chlorine, that makes blonde swimmers' hair go green. The acids in aspirin and water, as well as in tomato and lemon juice, can neutralize copper, helping get the green out.
To prevent further greening, before swimming rinse hair in cold water to tighten the cuticle, then put on conditioner and a swim cap. After swimming, wash hair with swimmer's shampoo, which binds to the copper on the hair to remove it.