When the sky turns green it means that most likely a thunderstorm is coming so when you see the sky turn green do be prepared for a thunderstorm.
Every time around here when the sky turned green a thunderstorm has occurred within 24 hours to sometimes within 4 hours of the sky turning green.
Then again sometimes the sky turns green and then it goes back to it's original color and then no thunderstorm happens.
The weather can change in an instant though so the skies can be one color and then change to another color and then can go cloudy and back to sunny on the same day.
A green color sky can also indicate that there may be a tornado along with the thunderstorm but it doesn't always mean there will be a tornado with the thunderstorm but it's good to be prepared.