Yes, jigs is right (or whoever it was). This was on the net
"Over the years I have often heard people speak about the sky turning green before a particularly bad storm. Sometimes those storms produce tornadoes. But the tornado isn't what causes the sky to turn green. The color magician is the hail in the thunderstorm. As light from above travels through the storm, it is reflected and refracted by the water droplets and hailstones. The hail allows more of the green to get through, giving the clouds that hue."
"Over the years I have often heard people speak about the sky turning green before a particularly bad storm. Sometimes those storms produce tornadoes. But the tornado isn't what causes the sky to turn green. The color magician is the hail in the thunderstorm. As light from above travels through the storm, it is reflected and refracted by the water droplets and hailstones. The hail allows more of the green to get through, giving the clouds that hue."