The Kellehers

Step 1: Open a blank layer above the image. Select the rock at lower left and invert the selection (Ctrl-Shift-I). (In CS3, the quick select tool works best for selecting the rock.) Now you’ve got everything except the rock selected.

Step 2: Click the layer mask icon at the bottom of the layers palette (the little square with a white circle in it). This creates a mask that covers the rock but leaves the sky revealed. Open the drop-down menu at the top left of the layers palette and change blend mode to lighten.

Setp 3: Open the color picker and sample the lightest blue in the lower right corner, where the yellow circle is. This sets that color as the foreground color.

Step 4: In the layers palette, click on the blank layer to the right of the name “Layer 2″ to deselect the mask. (When the mask is active, the mask icon has a white frame around it. When the layer is active, the mask icon has a black frame around it. You want a black frame.) Fill the layer with the sampled color (Ctrl-Backspace).

Why this works: When the blend mode is set to lighten, the blue fill will affect only those color that are the same lightness or darker. Since the clouds are lighter, they aren’t affected — except for cloud edges where the original dark blue fades into white. You lose some of those edges and the smaller clouds, but the overall sky color becomes uniform. Note that it’s important to mask out anything that you don’t want the fill color affect.