The envSky texture also comes with settings for a Sun and Clouds, which can be animated using keyframes. The colour of the sky is easier to texture as well, allowing for us to create a red effect.
One thing which the tutorial failed to mention, was that in order to produce any clouds, I had to configure some of the settings in the cloud layer, namely the Contrast, Amplitude and Ratio. The tutorial just stated that I needed configure the Density settings under the Cloud Attributes of the envSky texture.
I have also done an animation test so I can see the sunset in action. It is very quick but it demonstrates how I can reduce/expand the number of clouds, along with change the colour of the sky through the course of the sunset/sunrise.
The texture of this sky is cloudier and conveys a more open space than a cel shaded sky.