Why does the sky take on a metallic green appearance when tornadoes or severe thunderstorms are imminent?

Dear Tom,
Why does the sky take on a metallic green appearance when tornadoes or severe thunderstorms are imminent?
— Charisse Aguilar, Wood Dale

Dear Charisse,
Various explanations exist for the greenish color that sometimes appears in thunderstorms, but it’s still an unanswered issue. One thing is certain: The greenish color is not associated with severe thunderstorms or tornadoes. The existence of hail, once offered as an explanation, has been ruled out. Reflection of light from vegetation has similarly been eliminated: Some green thunderstorms have been observed over vegetation-free areas. A widely held explanation is that sunlight filtered through the deep layers of a thunderhead have much of the light absorbed, especially late in the day or early in the morning, leaving green as the main color to come through.