Understanding what you’re seeing in each channel gives you the know-how to create complicated selections and fine-tune your images. In this tutorial, you’ll look inside the different color channels, beginning with the most common color mode: RGB.
Unless you’re preparing an image that’s headed for a commercial printing press (as opposed to the ink jet printer you’ve probably got at home) and they require you to use CMYK, RGB mode is the place to be. After all, your monitor is RGB, as are your digital camera and scanner.
Photoshop doesn’t display individual channels in red, green, and blue—they’re in grayscale so you can easily see where the color is most saturated. Because colors in RGB mode are made from light, white indicates areas where the color is at full strength, black indicates areas where it’s weakest, and shades of gray represent everything in between.
Each color channel contains different information:
This channel is typically the lightest of the bunch and shows the greatest difference in color range. This channel is very light because there’s a lot of red in the woman’s skin and hair. Also, this channel can be very important when you’re correcting skin tones. You can also run a blur filter on this channel to instantly soften skin.
You can think of this channel as “contrast central” because it’s usually the highest in, well, contrast. (This makes sense because digital cameras have twice as many green sensors as red or blue ones in order to mimic the human eye, which is most sensitive to green light.) Remember this channel when you’re creating an edge mask for sharpening or working with displacement maps.
Typically, the darkest of the group, this channel is super useful when you want to create complex selections in order to isolate an object. It’s also where you’ll find problems like noise and grain.
Though you’ll probably spend most of your time working with RGB images, you may also need to work with images in CMYK mode. In order to produce true black, grays, and shades of color (colors mixed with black to produce darker colors), the folks who ran printing presses added black as a fourth printing ink.
They couldn’t abbreviate it with B because they were afraid it’d be confused with blue (as in RGB), so they used K instead—as in “black.” That’s where the abbreviation CMYK comes from. Its name stands for the cyan, magenta, yellow, and black inks commercial printing presses use in newspapers, magazines, product packaging, and so on. It has a composite channel at the top of the Channels panel named CMYK. You can pop into this mode by choosing Image>Mode>CMYK Color.
A professional printing press separates the four CMYK channels of your image into individual color separations. Each separation is a perfect copy of the color channel you see in Photoshop, printed in its respective color (cyan, magenta, yellow, or black). When the printing press places these four colors atop each other, they form the full-color image. This technique is known as four-color process printing.
Because CMYK channels represent ink rather than light, the grayscale information you see in your Channels panel represents the opposite of what it does in RGB mode. In CMYK mode, black indicates color at full strength and white indicates color at its weakest.
Creating a high-key portrait effect
Even if you’re not sending your image to a printing press, you can still have some
fun in CMYK mode. For example, you may have noticed that the Black channel looks pretty darned neat. It resembles a popular portrait effect called high-key lighting, in which multiple light sources are aimed at the subject to create a dazzling image with interesting shadows. Some folks labor long and hard to achieve this look in Photoshop when they could simply resort to a bit of channel theft instead. To create this effect, just extract the Black channel from a CMYK version of the image. Here’s how:
Step 1. If your image is in RGB mode, make a copy of it by choosing Image>Duplicate.
Because you’re going to change color modes in the next step, it’s a good idea to do that on a copy of your image since you lose a bit of color info when you switch from one mode to another. If your image is already in CMYK mode, skip ahead to step 3.
Step 2. In the duplicate image, choose Image>Mode>CMYK Color. If your document includes more than one layer, Photoshop asks if you want to combine them into one by flattening the image (compressing all layers into one). If those additional layers affect the way your image looks (for example, Adjustment layers), click Flatten. (Flattening a document is typically a scary move, but here you’re working with a duplicate of your original document.) If you see another dialog box asking about color profiles, just click OK. For now, all you need to know is that, since you’ll end up back in your original RGB document in step 5, the CMYK profile won’t affect anything.
Step 3. In the duplicate image’s Channels panel, activate the Black channel. When you click the Black channel to activate it, Photoshop automatically turns off the other channel’s visibility.
Step 4. Copy the Black channel. Press ⌘-A (Ctrl+A on a PC) to select everything in the Black channel, and then copy it by pressing ⌘-C (Ctrl+C).
Step 5. Switch back to your original RGB document and paste the Black channel into the Layers panel.
Click the original RGB image’s tab or inside its window. Next, open the Layers panel by clicking its icon in the panel dock or choosing Window>Layers. Then paste the Black channel by pressing ⌘-V (Ctrl+V). (You don’t have to create a new layer; Photoshop does that for you.) The beauty of producing this effect inside your original RGB document is editing flexibility. For example, you can lower the layer opacity of the new Black layer for a completely different, yet interesting, partial high-key/partial color effect.
Step 6. Close the duplicate document because you don’t need it anymore. That’s it! You’ve now got yourself a beautiful, high-contrast look that took only minutes to achieve.
Lab mode separates an image’s lightness values (how bright or dark it is) from its color information. This mode gets its name from the channels it includes: Lightness, a, and b. Lab mode isn’t used for output like RGB and CMYK modes; instead, it’s useful when you want to alter an image’s lightness values—when you’re sharpening or brightening, for instance—but not its colors. (Likewise, you can adjust the color information without affecting the lightness values to, say, get rid of a color cast.)
You can pop into Lab mode by choosing Image>Mode>Lab Color, and if you peek at the Channels panel, you’ll see x-rayish images like the ones in the image below.
Here’s what’s in each channel:
- Lightness. This is where you’ll find the details of your image, minus color; it looks like a wonderful black-and-white version.
- a. This channel contains half of the color information: a mixture of magenta (think “red”) and green.
- b. Here’s the other half: a mixture of yellow and blue.
Unless you’re preparing an image for a commercial printing press, you’ll never use this mode—although you may switch to it accidentally. If you delete one of the color channels in an RGB, CMYK, or Lab mode document, Photoshop plops you into Multichannel mode without even asking. If that happens, just use the History panel to go back a step or press ⌘-Z (Ctrl+Z on a PC) to undo and bring back the channel you accidentally deleted.
Multichannel mode doesn’t have a composite channel; it’s strictly for two or three-color print jobs. So when you enter this mode by choosing Image>Mode>Multichannel Photoshop converts any existing color channels to spot channels. When you convert an image to Multichannel mode, Photoshop promptly does one of the following (depending on which color mode you were in before):
- Converts the RGB channels to cyan, magenta, and yellow spot channels.
- Converts the CMYK channels to cyan, magenta, yellow, and black spot channels.
- Converts the Lab channels to alpha channels named Alpha 1, Alpha 2, and Alpha 3.
- Converts the Grayscale channel to a black spot channel.
These radical channel changes cause drastic color shifts, but you can edit each channel individually both its contents and its spot color—to create the image you want. When you’re finished editing, save the image as a Photoshop (PSD) file by choosing File>Save As and picking Photoshop from the Format pop-up menu.
The rest of Photoshop’s image modes aren’t very exciting when it comes to channels: They each have just one. They include Bitmap, Grayscale, Duotone, and Indexed Color mode (the latter is used in GIF files). Photoshop has one other type of channel: alpha channels. Their job is to store selections so you can use or edit them later. But a detailed discussion about it, we’ll have in a future tutorial.