There are two primary color modes in use: RGB and CMYK. RGB (Red, Green, Blue) is the standard archival and delivery color space, and holds more color information than CMYK (Cyan, Magenta, Yellow, Black) which is used for print output.
The conversion of an RGB file to CMYK involves a loss of color information that is not reversible, so in general it is best to retain images in RGB as far as possible in the workflow. CMYK's image files with 4 channels are 25 percent larger than the equivalent RGB with 3 channels. Adobe RGB (1998) is the most common delivery standard, with ECI RGB being adopted by some publishers in Europe. Unfortunately, the sRGB color space is probably used more often than either, although its smaller color gamut clips some colors that fall within the gamut of an offset press rendering those otherwise printable colors un-printable.
Color profiles specify which color space the image is in, and should always be attached to the image when saving, and preserved when opening, for correct viewing and handling. It is impossible to interpret the color numbers in an image correctly and with certainty if the color space is not known.
rgb or cmyk for file delivery?
The two color modes offer a choice of RGB or CMYK file delivery by photographers. Which is best depends on a number of factors:
- General publishing or high-end advertising/printing.
- The skill and pre-press knowledge of the photographer.
- Whether the printer/press/press profile is known in advance.
- Whether the photographer is delivering a final finished image, or whether there will be other color edits or retouching that will be done to the image by a graphic designer, retouch artist or other.
- The skill and pre-press knowledge of the graphic designer and/or the printer.
- The likelihood that embedded RGB profiles will be honored.
RGB delivery, and maintaining the files in RGB until just before inclusion in the page layout or even by means of the platesetter's or imagesetter's RIP is referred to as "late binding" workflow. The advantage of this workflow is flexibility. Image files can be worked on by people down the workflow, and the image files can be easily repurposed for different types of print output, or even used for display or web use. The caveat is that everyone in the workflow needs to understand and use color management and correct file handling and output sharpening. Another potential disadvantage- but one many clients have some familiarity with- is that the quality of the final output is not known until you are at the proof, or even the press stage. With that as an introduction, here is a chart showing the main differences between the two workflows, followed by information about the RGB workflow, followed by information about the CMYK workflow.
| RGB | CMYK |
| Image will pass through a number of hands on its way to output | Shorter supply chain to design, or print |
| Image for repurposing — print, web, presentation etc. | Use in print only |
| The default for general publishing use | An option in controlled high-end colour critical situations |
| Always use when the image is to be archived for further use | Not suitable for archiving |
| Larger gamut than CMYK for most colors | Color range restricted for print output |
| Smaller file size than CMYK — 3 channels, R,G, and B | Larger file size due to 4 channels, C,M,Y and K |
the rgb workflow
The most commonly used RGB profiles include:
- sRGB, a narrow-gamut but common space.
- Adobe RGB (1998), a wider-gamut color space.
- ECI-RGB, a color space increasingly popular in Europe.
Adobe RGB (1998) and ECI-RGB come much closer than sRGB to encompassing the color gamut of standard CMYK profiles. Those involved in high-end, fine-art inkjet printing often use ProPhoto RGB, which has an even wider gamut. But the wider gamut leaves more room for error, making it problematic for standard delivery. Note, too, that proper display of colors in Adobe RGB (1998) files requires calibrated monitors and users who preserve embedded profiles. Under ideal circumstances, it's best to work with the Adobe RGB (1998) or ECI color space, but the additional color gamut can become a liability if the printer strips out or ignores the embedded profile, resulting in flat and toneless images. Despite these problems, it is important to communicate that the default setting in Europe and US Pre-press defaults in Photoshop is currently Adobe RGB (1998). Everyone in the workflow should be encouraged to implement color-managed workflows for best results.
general principles for rgb file delivery
- If no submission guidelines are provided, the standard submission format is Adobe RGB for print or general use, and sRGB for web only use.
- Photographers should always embed the RGB profile.
- Photographers state clearly what they are providing in the read-me file.
- Receivers of RGB files should always preserve embedded RGB profiles when opening files.
- If the client asks for sRGB for print, then that is what the photographer has to provide. The results are the responsibility of the client.
For web and digital projection, sRGB is the clear choice. The web is a lowest common denominator medium, where you must assume monitors are not calibrated and web browsers are unable to read color profile tags.
For more information about web browser color management, visit these web pages:
http://www.northlight-images.co.uk/article_pages/web-browser-color-management.html*
http://regex.info/blog/photo-tech/color-spaces-page2/*
the cmyk workflow
The most commonly used CMYK profiles include:
- US Web Coated (SWOP) v2, ships with Photoshop as the North American Prepress 2 default.
- Coated FOGRA27 (ISO 12647-2-2004), ships with Photoshop as the Europe Prepress 2 default
- Japan Color 2001 Coated, the Japan Prepress 2 default.
- Many printers in the US are starting to use the G7 methodology and GRACol CMYK profiles for their sheetfed presses. These new CMYK profiles will likely ship in the new versions of Photoshop.
Although the industry trend is for the RGB workflow to extend further towards the print output, the vast majority of small- to medium-size offset printers still prefer receiving CMYK files, even if they often can't specify exactly which CMYK profile they need. As result, many graphic designers use Photoshop's Image>Mode>CMYK to convert RGB files to CMYK. By doing so, they convert to the default CMYK profile in Photoshop's Color Settings. The initial default setting is U.S. Web Coated (SWOP) v2, or Coated FOGRA27 in Europe, even though these may not be optimal profiles. Graphic designers need to assess the choice of CMYK delivery based on the capabilities (or lack of them) of the photographer or the printer to convert files. In some high-end situations, the photographer may be skilled at CMYK conversion and will achieve better results than the designer. In these instances, the photographer will need access to the printer settings. Our testing and other anecdotal evidence leads us to believe that U.S. Web Coated (SWOP) v2 in the U.S. at least, is much preferred over the U.S. Sheetfed v2 profiles even for sheetfed presses. This is because most printers have standardized on the SWOP profile and not the Sheetfed profile. Printers that have gone over to G7/GRACoL printing are finding that while the GRACoL profiles are optimal, the SWOP profiles still work quite well, which was an intended result of the G7 method. One additional, but little known fact is that many printers routinely re-separate CMYK files to better match their press conditions, or even to save ink by trimming total ink coverage and/or increasing GCR (grey component replacement). Some printers use Photoshop to do this, which while not optimal, works, while others use device link profiles, a better method.