This amazing action usually just changes the sky for you, with no selection necessary.
But it can sometimes alter other bits (notice the doorways) which may need masking out.

The effect is not dramatic, but in keeping with the original picture.

When you have worked out why this very simple technique works all the time, you will begin to realise the benefits of inspecting channels before you start playing with images.
You will also find that you begin to see how you may solve problems by using channels, or aspects of them, that had never occurred to you before - and umph - you are off on a new plane of perception which is great great fun. 

So good luck with this one!