As you may recall, you can specify in Photoshop CS what type of image mode you want to use. If you choose Image-> Mode-> RGB Colour from the menu bar, the image will be displayed in RGB mode. The items in the channels palette will reflect the mode that you are working in.

 

If you examine the Layers palette you will see a tab labelled Channels. Clicking on this tab will display the Channels palette. You can also use the Window menu to access the Channels option to display the same palette.

 

If you are working in RGB colour mode, the channels palette will display three separate channels for the red, green, and blue information respectively.

 

Beside each of these channels, you will see an eye icon which you can click to hide or display the given channel.

 

 

If you hide the red channel, the image will be displayed with greens and blues only. If you hide the blue channel, the image will be displayed in reds and greens. Though this in itself can create some interesting effects, it is only scratching the surface of what you can do with channels.

 

You may notice that when you hide two of the three colour channels, the image turns into a black and white (more accurately called greyscale) image. The shades of grey in the image will change depending on which two colour components you hide. (Photoshop requires that at least two colour channels be visible if the image is to have any colour.)

 

The top item in the channels palette will be labelled RGB. If you make this item visible, you will make all of the channels visible. The channels palette lets you approach each colour channel individually. If you click a channel item in the channels palette, it will become the active channel (something like the layers palette).

 

If you are working in CMYK mode (Image -> Mode->CMYK Colour), the channels palette will contain cyan, magenta, yellow, and black channels. Just like RGB mode, you can hide or display a colour channel by clicking its eye icon.

 

As you probably know, CMYK is the colour representation used for printing documents. Your printer will add varying amounts of cyan, yellow, magenta, and black together to provide coloured prints. The channels palette lets you approach these colours on an individual basis. If you click an item in the CMYK channels palette, it will become the active channel.

 

 

If you are working in Lab colour mode (Image->Mode->Lab colour), the channels palette will reveal one luminance channel and two chrominance channels. The luminance channel (or lightness channel) is labelled L and the two chrominance (colour) channels are labelled a and b. The a channel represents a combination of greens and reds, and the b channel represents a combination of yellows and blues. Once again, you can hide or display a channel by clicking the eye icon next to it, and you can display all channels by clicking the eye icon square by the item labelled LAB. (If any channels are hidden, there will not be an eye next to the LAB item in the palette.)

 

Here is the Channels palette for an image in Lab colour mode.

 

 

As with the other colour modes, the channels palette allows you to access each LAB channel individually. If you select an item in the LAB channels palette, it will become the active channel.

 

Whether you are in RGB mode, CMYK mode, or LAB mode, the active channel will be highlighted in blue, while the rest will not be highlighted. If you select the top item in the channels palette (the combination of all channels), all of the channels will be highlighted. When you select an individual channel, you can perform image adjustments on only that channel by choosing from the Adjustments options under the Image menu. This will allow you to fine tune the light and shadow, the saturation, the contrast, or other image qualities on a channel by channel basis.

 

You can create a new empty channel by clicking the Create new channel icon at the bottom of the channels palette. You can also duplicate a channel by dragging it down to the Create new channel icon.

 

 

As an example of manipulating channels, consider the screenshot shown above. In the image on the left, the red RGB channel was adjusted for shadows and highlights. In the image on the right, the green RGB channel was has the same adjustments for shadows and highlights. Notice how the same adjustments give different results when performed on different channels.

 

Note: The adjustment options available in the image menu may differ between colour modes, or depending on what channel is active.