As we know, The Thunderstorms,which can be the home of tornadoes tend to happen late in the day.

Before a thunderstorm,golden-reddish light from the sun glow in the sky, and a natural bluing effect of the air when combine,they create a green sky.

The storm provides a dark backdrop and offsets this greenish  hue.

Relation between colour of sky and the tornado-

The green sky is significant but not proof that a tornado is on the way.

A green  cloud will only occur if the cloud is very deep, which generally occurs only in thunderstorm clouds.

So, we can say green sky is a warning sign for upcoming tornado.