Darker

The sky grew darker and darker.

The sky grew darker and darker, and the wind blew harder and harder.

The sky became darker and darker.

The clouds are getting darker.

The clouds are getting darker; it's going to rain.

It is getting darker. It may rain soon.

Nancy is a shade darker than Helen.

Nancy is a shade darker than Helen is.

It is getting darker and darker.

As time went on, the sky grew darker and darker.