[AllenG (OP)]

I live in Georgia, USA.

During thunderstorms, it is a normal dark gray, but often during tornado  warnings the sky is an eriy green.
What would be the cause?

Many thanks,

--Allen

[paul.fr]

(I think) this is due to the state of the cb cloud and the volume of hail / graupel within it, this scatters the light and produces the greenish / blueish colour. Obviously, you want to be somewhere else when you notice the sky turn green!