Colour spaces are the source for a lot of confusion. This is a rewrite of an earlier explanation and is aimed at practical usage for post-processing and distribution of your images. The intention here is to cover just the important points. Links for more information can be found at the bottom.

A RGB colour space (called Working Space when editing the file in Adobe Photoshop) gives definition to an image’s values (or coordinates) within that space. Both the values (e.g. 255,0,0) and space (e.g. sRGB) are required to define the actual colour. A RGB file provided without specification of its colour space isn’t useful.

The following shows how to tag your file with its colour space in Adobe Photoshop’s Save As dialog:

The colour space is a container, the size and shape of which limit the range of colours (gamut) which it defines. The larger the space, the more extreme the colours can potentially be but with correspondingly less colour resolution (precision). In general, the ideal colour space is the smallest space that will just contain all the colours of the rendered image and maintain maximum colour resolution.

A colour space is associated when the image is captured or rendered. If shooting JPEG it is set by camera settings (usually sRGB or Adobe RGB (1998)). When rendering RAW files by the software itself (from a list of choices). When scanning, by the scanner software. Once a colour space has been set for an image, in general it should not be changed as doing so will decrease colour resolution. There’s nothing to be gained by having all your images use the same space as the software will adapt to the colour space tagged to each individual file. Using the same colour (or working) space for everything is bad practise.

The following are recommended Adobe Photoshop Color Settings to avoid unwanted colour space conversions:

When presented with the choice of which colour space to use (at time of capture or rendering) you need to assess the range of colours in your image. This requires some experience. If you choose too small a space, some of the original image colours will be “clipped” to the colour space boundary. If you choose too large a space, you will potentially lose colour resolution. If you select a space larger than Adobe RGB (1998) or eciRGB_v2 it is imperative that your image also be in 16-bit mode and stays so for the life of the file (16-bit mode should also always be set when converting between spaces).

The following are colour space usage guidelines (in increasing order of size):

sRGB is sufficient for most natural colours (some flowers and bird plumage excepted). It is ideal for portraits as it will maintain maximum colour resolution, namely subtlety of skin tones. sRGB uses a modified 2.2 gamma.

Adobe RGB (1998) or eciRGB_v2 are medium sized spaces which encompass most (but not all) of the colours that can be printed. As such they are recommended for general use. eciRGB_v2 is preferable to Adobe RGB (1998) as it is a better match for the gamut of paper/ink used by Macquarie Editions. It also uses L* instead of 2.2 gamma. (L* is the Lightness axis of L*a*b* and modelled on how we perceive tonality.)

Macquarie Editions RGB v4 is a larger space (modelled on Beta RGB) which encompasses all the colours that can be captured by transparency film and used by reference targets. It uses an ideal L* gamma. In general this is the largest space you should ever need. (Note that its gamut exceeds that of even the best monitors.) Use only in 16-bit mode (and don’t save as JPEG).

ProPhoto RGB is an extremely large space, a significant proportion of which cannot be printed or even seen by the human eye. Its coordinates correspond to the internal space used by Adobe Photoshop Lightroom, but with a 1.8 gamma. It can be useful if you’re going to bounce your image between Adobe Photoshop and Adobe Photoshop Lightroom. Use only in 16-bit mode (and don’t save as JPEG). (Don’t be influenced by the “Pro” in ProPhoto. In many cases, sRGB can be a more appropriate space to use.)

 

To sum up:

• A colour space is required to define the actual colours.
• If your image is in a colour space (whatever it may be), leave it in that space (namely don’t convert it).
• If presented with a choice of which space to use, select the smallest space appropriate to that particular image.
• Always use 16-bit mode when your image is in a larger space. If your computer is powerful enough and you have sufficient storage space you can use 16-bit mode for everything (this can be useful to prevent posterisation/banding).

 

More information:

International Color Consortium
Information About RGB Working Spaces