For the purposes of this discussion, we're going be using this simple image of an apple, which can be downloaded for free over at Dreamstime.
Channel Basics
Now, we'll start with some of the true fundamentals. First, you can access your channels by opening up the Channels Palette (Window - Channels from in the menu). This is where we will be doing most of our work, both in this article and in the follow up tutorial.
In this case, we can see four channels - RGB, Red, Green and Blue. By the way they are laid out in the palette, they actually look a lot like layers. Don't be confused, though. That is strictly a matter of interface. Channels and Layers are entirely different entities.
You may also notice that the channels seem to reflect the current colour mode of the document - which is RGB. This is indeed true, and leads us to another important point. The Channels Palette will change based on the colour mode of the document. This is because each colour mode uses a different method for defining colour, and at their most basic level channels are simply a programatic way of displaying the interaction of basic primary colours, which combine to create a much larger range of full colour.
The Big Two: RGB and CMYK
I'm sure that many of you are familiar with basic concepts of how colours are mixed, but since this is one of the fundamental building blocks of how channels work, let's quickly review our two basic colour structures.
First, we have RGB, which is the colour mode of all things like your monitor, television and various handheld devises. RGB makes use of concepts of additive colour, combining tiny bits of red, green and blue lights in varying degrees in order to create a full and rich colour gamut.
On the other hand, we have CMYK, which is what is used for print. With this kind of colour, a printer or press lays down tiny dots of cyan, magenta, yellow and black inks, which blend together to create the illusion of colour. While pixels on a monitor actually emit coloured light, the tiny dots on a page absorb coloured lights, reflecting only the colour that you see. As more ink is mixed together, more light is absorbed. That's why it is called subtractive colour - because less and less gets reflected as you add more ink.
I should also note that when you're working in CMYK your screen is still displaying in RGB - Photoshop is simply approximating CMYK colour for display. If you want to learn a bit more of the science, read about additive colour and subtractive colour on Wikipedia.
For our purposes, the important thing to know is that each of these colour models require different sets of colour information, which are reflected in their types of channels. Let's examine this a bit more closely.
First, open up the image and then pop open the Channels palette. When downloaded from the internet, the image should be in RGB mode by default, so you should see four different channels. Actually, there are three channels (Red, Green and Blue), and then an automatic, and uneditable channel (called RGB). Each channel visualizes the three different streams of colours that compose the document, which we can see by taking another look at our previous screenshot:
The Photoshop default is for each channel is actually represented by a simple greyscale image (though we'll look at how to change that). For example, here is the red channel for our apple:
It may seem odd that the apple itself actually seems washed out, rather than strong and vibrant, which you might expect given that the fruit itself is also red. There is a good reason for this. Within an RGB channel, any white pixel will be at its maximum brightness for that colour - red in this case. In other words, it will emit the maximum amount of red light. Conversely, wherever it is black, the pixel will not emit any light of that colour. So, because the apple is red, pixels that make up that part of the image are actually quite light.
Conversely, if we convert the apple image to CMYK, we will see something quite different. We now have five channels, one for cyan, magenta, yellow and black, as well as the automatic combined channel.
In this case the channels work somewhat differently, because we are working in a colour space that is intended to mimic the workings of ink on paper. As such, you will find that wherever a pixel is white, it is meant to represent blank, unprinted paper. Wherever a pixel is black, it is meant to represent an area where the paper is covered with the maximum saturation for that particular colour.
Let's look at the magenta channel as an example. To create red with ink, we can actually combine magenta with yellow. So, if we printed our apple image, we would expect there to be a lot of magenta ink used. This is accurately reflected in the magenta channel.
Notice that the areas of the deepest red are the most saturated with black, whereas areas that contain relatively little red (such as the reflections and the yellow area near the top of the apple) remain much whiter.
That's really the basics of what channels are. They are simply a way of splitting apart the different colour information in a document, giving you the ability to make tweaks and adjustments on a colour by colour basis. A quick word of warning, though: using channels to make extensive colour modifications on a heavily layered document can be a bit tricky. Making a change in a channel will still only effect the current layer, so I would recommend doing some experimenting.
In fact, I always recommend experimenting! I've found that it's the absolute best way to learn.
Seeing Channels in Colour
Personally, I prefer working with the default greyscale rendering of the channels. However, if you're finding it difficult to wrap your head around how the colours are actually working, Photoshop has a little interface option to help make it easier for you. Select Photoshop - Preferences - Interface from the menu (PC: Edit - Preferences - Interface). In the dialogue box you should see a little check box with the option "Show Channels In Colour". By default this is turned off, but try turning it on. In RGB mode, the channels palette and red channel should look something like this:
In CMYK mode, the channels palette and magenta channel should look something like this:
This little trick can be super helpful when it comes to visualizing the colour behind unique channels. However, when it comes to using them for extractions, it can make things a little bit more difficult. So, in the next article, I will have this option turned off again
