Depending on the colors in the image, it is often possible to easily make a complex selection by using one of the image's channels; Red, Green, or Blue in an RGB file and Cyan, Magenta, Yellow, or Black in a CMYK file.

For instance, in the example image below most of the blue is in the sky so if one wanted to darken the sky (usually a good thing to do) the Blue channel of the RGB file could be used to almost effortlessly select the sky and that selection could then be darkened.