If you accidentally press "Ctrl-2 -- "Cmd-2" on a Mac -- while you view or work in a color file in Adobe Photoshop CS3 or earlier, don't panic when your image suddenly looks like a black-and-white photograph. Nothing's happened to your file. The keyboard shortcut you typed tells Photoshop to hide some of your image's color information.
Open the "Window" menu and choose "Channels" to reveal the Channels panel, which shows the individual channels in your file: red, green and blue in an RGB document; and cyan, magenta, yellow and black in a CMYK document. Only one channel in the panel shows an eyeball icon in the column to the left of the channel names, indicating that only that specific channel's image information remains visible. In an RGB file, the temporarily grayscale image information you're looking at comes from the Green channel.
Press "Ctrl-~" to return to the composite, or full-color, view of your file. That squiggly character in the keyboard shortcut, called a tilde, appears to the left of the exclamation point on the top row of your keyboard. To use your mouse instead of the keyboard to return to the composite view, either click in the column in front of the name of each channel whose eyeball is turned off, or click in front of the name of the composite channel.
Examine your image again and you see its normal appearance. Hiding color information didn't delete it from your file.
Experiment with turning individual channels off and back on again to see how their color information looks when you view one channel at a time or combinations of channels. Click on channel eyeballs in the Channels panel to turn visibility off and on.
Examine the Channels panel closely. The individual channel-specific keyboard shortcuts appear to the right of the channel names. If you memorize only one of them, make it the shortcut that restores composite view.
You don't need to open the Channels panel to switch among channels from the keyboard.
The same keyboard shortcuts work regardless of whether your file is in RGB, CMYK, Lab or any other color mode. Combine the "Ctrl" key with numbers up to 9 to access color channels or alpha channels.
In Adobe Photoshop CS3 and later, Adobe Systems changed the default keyboard shortcuts that switch among image channels and for returning to the composite view of a file. In these versions, press "Ctrl-2" to switch to the composite. Add 1 to the earlier-version shortcut numbers for color channels.