Today’s Question: I know you’ve covered this before, but can you remind me how to create a selection from a color channel in Photoshop?
Tim’s Quick Answer: You can create a selection based on a color channel by first duplicating the applicable channel, increasing contrast for that channel, and then cleaning it up as needed. You can then load a selection based on the duplicate channel you worked on.
More Detail: In many cases creating a selection based on one of the three color channels (red, green, or blue) can provide a quick way to isolate a specific subject in a photo. Even in situations where there isn’t much contrast in tonality, even a small amount of color contrast can enable you to create a good selection quickly, and then refine as needed from there.
To get started, go to the Channels panel (you can choose Window > Channels from the menu if the Channels panel isn’t visible) and decide which channel contains the best contrast for the area you want to select. Then drag the thumbnail for that channel to the “New Channel” button (the blank sheet of paper icon) at the bottom of the Channels panel.
Next, apply a Levels adjustment to maximize contrast to the extent possible for the duplicate channel you created. Start by choosing Image > Adjustments > Levels from the menu. Then in the Levels dialog drag both the black point and white point sliders below the histogram inward in order to maximize contrast to define the subject you want to isolate. For example, if you were trying to isolate the sky you would want to drag the sliders inward until the sky is white and the rest of the image is black for your duplicate channel.
You can also perform some cleanup work at this point, such as by using the Brush tool to paint with black or white in order to clean blemishes within the areas you have defined through contrast on the duplicate channel.
When you’re ready to load a selection based on the duplicate channel you’ve refined, you can simply click the “Load Selection” button (the dashed circle icon) at the bottom of the Channels panel. Then click the RGB thumbnail to return to your full color image, and you can employ the selection as the basis of a layer mask for a targeted adjustment or composite image.