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Digital images are composed of pixels that specify RGB colors. A color space, such as sRGB or Adobe RGB (aRGB), defines how these pixel values are interpreted. Every device has a profile that describes the its color space. Recent cameras tag the EXIF information with the color profile used by the camera. There's a color profile that defines how your monitor displays colors. There's a color profile for your printer that specifies how pixel values get mapped to colors on the printout.
All inputs and outputs to Photoshop have color profiles. In addition you can choose a working color space for editing purposes within Photoshop (see Edit > Color Settings). When you open an image Photoshop will transform pixel values to your working color space. As you make changes to the image you will be working in this color space. To display images on a monitor Photoshop uses the monitor profile. To print the image on a printer Photoshop uses the printer profile.
Each color space has its own unique characteristics. One measure of a color space is the gamut or number of colors that can be represented. For example aRGB has a wider gamut than sRGB. This is accomplished by using a coarser granularity for density values. For example, the difference in color between (0, 128, 0) and (0, 129, 0) is more pronounced with aRGB than sRGB.
Usually sRGB or aRGB is chosen as a working color space. Color space sRGB is the standard for the internet and the default color space assumed by most applications. If you're emailing an image to a friend, or posting it on the internet, it should be in the sRGB color space. Some printers are capable of displaying a wider gamut of colors than sRGB can designate, so utilizing a wider-gamut profile, such as aRGB, as a working color space can be beneficial.
The following illustrates the difference between sRGB and aRGB when viewed by applications that are not color-space aware. Although both images appear identical in Photoshop, the aRGB image is a muted version of the original when viewed by a browser.
If you use aRGB as your working color space, you must manually convert images to sRGB before saving files for the internet or emailing to friends. The saving for the web menu in Photoshop (File > Save for Web) strips EXIF and color space information from an image but does not convert images to sRGB. To convert from aRGB to sRGB choose Edit > Convert to Profile (Image > Mode > Convert to Profile for older versions). Then choose File > Save for Web. Do not choose Assign Profile, as this assigns a profile without changing pixel values.
Labs that use the Fuji Frontier printer often recommend sRGB as a color space. Justification for this approach is apparent when you consider that the sRGB color space is a close match to the profile for the Fuji Frontier. For an interesting illustration of printer gamuts and camera color spaces visit Dry Creek Photo.
So for my purposes I shoot sRGB JPEGs and import to an sRGB workspace. My images are destined for friends, presentation on the internet, and printing on a Fuji Frontier printer at ezprints. All require the sRGB color space. It's a simple workflow and I'm quite satisfied with the results obtained. Specify working color space under Edit > Color Settings. Here is a screen snapshot of my color settings.
