Sometimes, when you’re cutting out skies or other uniform colors in a photograph, you can select the color from a channel. Look at the Channels panel and examine each channel separately. You should see a nice separation of light and dark tones. Duplicate the channel and then apply a Levels or Curves adjustment to further enhance the difference. All you need to do now is click back on the RGB channel, and then Command-click (PC: Ctrl-click) on the channel to load it as a selection.
This tip previously appeared in Colin Smith’s Photoshop Tips column, in the June/July, 2019 issue of Photoshop User magazine.