How to Use Channel Masks to Make Selections in Photoshop

RGB Channel Masking

Open the image in Photoshop. Just about any version of Photoshop that has layers should have channels.

Click on the Channels tab. If you don't see it, go to Windows >> Channels. The channels are RGB channels. All images are made of varying degrees of red, green, and blue.

Click on each of the channels to see which channel has the most contrast. Quite often, that is the blue channel, but it really depends on the colors of the image.

Drag the chosen channel to the plus sign at the bottom of the channels window. This will create a copy of the channels. You make a copy because if you change the actual channel, you will change your image in pretty unpredictable ways.

Click on the copy of the channel and press CtrlL. This brings up the Levels dialog window.

Move in the two sliders on the end. The black and the white (shadows and highlights) sliders. You are trying to get even more contrast.

Press down Ctrl while clicking on the layer that you just adjusted levels on. This will create a selection for a mask to be created with.

Click on the layer that you want to apply the mask to, hold down Alt and click on the Mask icon. This will automatically create an inverted mask of the selection. Alternatively, you can invert the channel in the Channels tab.

Remove the parts of the mask that you don't want. If there are parts of the mask that are exposing pieces of the image that you don't want it to, just use the brush and paint black on the parts that you don't want to show.

Press Alt and click on the mask to see what your mask looks like.

Press B for Brush and change the Mode to 'Overlay'.

Select White as your foreground color and paint over the image where you want the white to be for cleaning up your mask.

Select Black as the foreground color and paint over the image where you want the black to be.

Apply your effect.

HSB/HSL Channel Masking

Duplicate your layer that you want to create a mask for.

Go to Filter >> Other >> HSB/HSL.

Go to the Channels tab. If you don't see it, go to Windows >> Channels. There are your three layers. Red is the Hue layer. Green is the Saturation layer and Blue is the Brightness/Lightness.

Hold down Ctrl and click on the 'Green' layer. This will select the saturated colors in your image in their varying degrees.

Delete the layer that you ran the HSB/HSL filter on.

Click on the layer (or adjustment layer) that you want to apply the mask to and click on the mask icon. If you need to invert the mask for different effects, press CtrlI to get the opposite mask.