If you want to select apart of an image in function of a color characteristic you can use Color Range in the select menu, and you can choose in the following list of : reds, greens, yellows, cyans, blues, magentas, higlights, midtones, shadows and out of gamut.
It might be usefull, for instance, if you want to adjust the brightness of the shadows of your image, without touching to its bright part.
