I don’t have Richard’s book in front of me, but the way to select the colors is to click on the foreground color box, select white, click on the background color box and click the blue you want.
Select the pixels that make up the sky. When the sky is selected (look for the marching ants around it) create a new layer and apply the filter in the selected pixels in the new layer.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com