Blending channels is complex. It often produces additional unintended color effects that may require further correction, such as shifts in hue that aren’t uniform across the tonal scale. Blending channels is neither the simplest nor the most direct path to color adjustment, but in certain situations (files that are exceptionally problematic), it may be the best path. The resulting benefits can be dramatic.
There are several ways to blend channels: Channel Mixer, Apply Image, Calculations and using channels as layers. Let’s review the options in detail.
The Channel Mixer (Layer > New Adjustment Layer > Channel Mixer) blends percentages of channels within a single document. It can be applied as an adjustment layer, so corrections made this way can be changed or masked indefinitely. It can’t be used to blend channels from two documents. The Channel Mixer is an excellent choice for making global (the same percentage of channels for the whole image) color to black-and-white conversions. If you want to control black-and-white conversions locally (different percentages of channels for different image areas), use channels as layers instead.
The commands Calculations (Image > Calculations) and Apply Image (Image > Apply Image) can also be used to blend channels. With these two commands, you can combine any two channels, from different documents, from any layer, at any opacity, with most blend modes. With Apply Image, you target the channel you wish to change. With Calculations, you blend to create a new document, a new channel or a new selection. Neither Calculations nor Apply Image can be used as adjustment layers or layers; consequently, corrections you make with either of these features are made permanently to an image. With Apply Image and Calculations, you can take advantage of two less frequently used blending modes not found with other tools (Add and Subtract), but you can’t take advantage of four frequently used blending modes (Hue, Saturation, Color and Luminosity)—even if you use the Fade command.
For the greatest control and flexibility, use channels as layers. How do you do this? Copy any channel and paste it into any destination as a layer. (Target a channel by clicking on it; copy that channel (Select All > Edit > Copy); then target the master channel (RGB) and paste (Edit > Paste).) You can activate, deactivate, mask, change or replace this new layer indefinitely. Use Layer Styles (double-click on the Layer icon in the Layers palette) to determine Blend Mode, Opacity, Advanced Blending, to select which channel is affected, and Blend If options, to determine how This Layer affects the Underlying Layer or which values of the overlying layer affect the values of the underlying layer. What’s more, you get a dynamic preview of any changes you make while you make them. The adjustments you make are flexible, so you can remove them or fine-tune any of the settings in future editing sessions. You even can blend two or more channels first, as layers, and then use the resulting new layer to blend with the Background layer. By turning channels into layers, you can achieve everything that the other methods achieve and more.
One File, Many Channels
You may be surprised to find that every file has at least 10 channels to choose from. How do you get so many? Consider the file in different color spaces—RGB, CMYK and LAB. Convert a duplicate file into another color space, and you can use any and all of the resulting channels. In fact, you can choose between many, many more channels when you consider that when converting to CMYK, there are five different options for generating a Black plate (None, Light, Medium, Heavy and Maximum) with two styles for each with two Separation types (UCR and GCR). But for the vast majority of situations, I recommend you try to keep things as simple as possible and stick with the standard three.