Think of color "channels" as color "filters". Whenever Photoshop displays a color image on the screen, it knows which colors to display by shining a light through the filters. First it shines the light through the red filter (the red channel). If no amount of light passes through the filter, Photoshop knows to display red at a value of 0. If all of the light passes through the filter, Photoshop displays red at a full strength value of 255. If some lesser amount of light passes through, Photoshop displays red at a value somewhere between 0 and 255 depending on how much light passed through. Then it does the same thing with the green filter (the green channel), assigning green a value of 0 if no light passes through, 255 if all the light passes through, and some value in between if some but not all of the light passes through. Then it does it with the blue filter (the blue channel). When it's done, it knows what value to set for red, green and blue, and it combines them to create the color we see. It does this for every single pixel in your image, so if your image contains millions of pixels, as most photos from digital cameras these days do, Photoshop goes through this process millions of times just to display the image you see on your screen. See how much Photoshop loves you? Okay, so a moment ago I said Photoshop couldn't care less. Let's just move on.

The "filters" Photoshop uses are those three separate black and white images we saw on the previous page. Let's look again at the red one:

Okay, so how does Photoshop manage to use this black and white image as a red filter? Remember how I said that Photoshop assigns a value between 0 - 255 to red based on how much light passes through the filter? Well, how much light is able to pass through the filter all depends on how bright or dark that part of the black and white image is. Any areas that are pure black don't allow any light to pass through, which means that red will be set to 0 in those parts of the image. Any areas that are pure white allow the full amount of light to pass through, so red will be set to 255 in those areas. And areas that are some shade of gray, which usually make up most of the image, will allow varying amounts of light through depending on how light or dark the shade of gray is. In the image above, we can see that the brightest parts of the image are the bird's beak and chest, which based on what I just said should mean that those areas contain a lot of red in the full color version. Likewise, his back, wing and belly are very dark, so those areas shouldn't have much red in them at all, if any.

Let's look again at the full color version:

We said the beak and chest should contain lots of red, and sure enough, they do! We also said the back, wing and belly should appear to have very little if any red, and they sure don't look red to me.

Let's look again at the black and white image Photoshop is using for the green channel:

This black and white image contains a lot of lighter areas, which means there should be a lot of green in this photo. Oddly enough, one of the brightest areas in the image is along the sides of the bird's chest, and I don't remember that being green. Let's check this out by looking at the full color version again:

There certainly is a lot of green in the image, which explains why the black and white image had so many lighter shades of gray. If I look at the side of the bird's chest though, which was one of the lightest areas in the black and white image, it doesn't look green at all. In fact, it's very much yellow! How is that possible? Simple. Red and green combined make yellow, so in order to display yellow, Photoshop has to mix red and green together.