Setting the screen color pulls a key, or in other words, creates a matte - the Screen Matte. Setting the screen color also despills the foreground, although you can also use the Despill Bias to remove more spill. In some cases this is enough to get a decent key. For more information on Screen Color, see .
The image below shows a well-lit blue screen behind an actor.
You should note that repeatedly picking colors does not add to previous selections and key more of the image with each click. To key more of the image, try picking different shades of blue then use the screen strength parameter. See Keying More.