Duplicated image layer set to multiply to darken the threatening sky. Mask the foreground.
Paint with light layer to add highlights to rocks
Stamp visible layer changed to Color Dodge to intensify rock colors, Mask background and lower opacity to taste.
forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=24196101&changemode=1
original image by redbaron41432: