When to Convert an Image from RGB to CMYK

Several years ago I wrote a short PIE Quick Tips article describing the difference between RGB and CMYK color and noting that you must convert RGB images to CMYK images prior to submitting a job to your offset or digital print provider. Following is an elaboration on this important topic.

Color on the monitor is created with light (red-green-blue, RGB, or additive color). The presence of equal amounts of all colors creates white light. In contrast, color on a printing press is created with ink (cyan-magenta-yellow-black, CMYK, or subtractive color), and the presence of equal amounts of CMY yields K (black, or, more precisely, a dirty brown). Additional black ink is then added to achieve a true black.

If you are preparing art files for final offset or digital printing, you will normally scan images, or open scanned images in Photoshop, into the RGB color space. You will then convert these images to the CMYK color space prior to handing off final art files to your offset print provider.

The big question is, when do you make the conversion and why?

RGB is a much larger color space than CMYK. Another way of saying this is that your color monitor can reproduce many more colors than can be printed on press. Therefore, you have more color image "information" to work with if you keep the photo in RGB format for as long as possible. The files are also smaller, interestingly enough, since you are defining color in three channels rather than four. It is also easier to achieve color balance in RGB color space than in CMYK format, since a neutral color contains equal amounts of the three hues (which is not exactly true in the 4-color--CMYK--color space).

Long books have been written to explain all the nuances of color theory and color conversion, but the most reliable rule of thumb is to make all corrections to your images within the RGB color space. Then, convert the images to CMYK prior to placing the TIFF files in your page composition document (an InDesign file, for instance). That is, color correct the image, fix any flaws, remove any dust, etc., in RGB mode, then convert to CMYK, and then review the image on-screen in CMYK mode.

Another benefit of doing this (particularly in this era of repurposing content for the web as well as print) is that you will also have a good RGB image to include in your web version of the job. (It will have been optimized for the RGB color space and therefore will be an ideal image for a video screen.)

You may notice, when you convert an RGB image to a CMYK image, that the colors change slightly. This may or may not happen, depending on the colors involved, and a photograph will often be more forgiving than solid areas of color in a graphic image (an illustration, for instance).

Why does this occur? The RGB color space includes hues that don't exist in CMYK color space. Both RGB and CMYK are slightly different subsets of the visible light spectrum (i.e., they don't overlap exactly). Therefore, your computer software automatically maps (or shifts) the "out-of-gamut" RGB colors that have no CMYK equivalents to the nearest color equivalent in CMYK color space. Sometimes this is fine. Sometimes it causes a visible color shift. (As an aside, Photoshop will let you identify and highlight these "out-of-gamut" colors, so you will know before printing exactly which colors may be problematic. Then you can choose to either accept the color conversion results or alter the colors.)

If you are concerned that the results of your conversion will not be satisfactory, you can always hand off the RGB image files (and the ICC color profiles) to your offset print provider for color conversion. (The ICC profiles are descriptions of the color capabilities of each device—such as your scanner, monitor, etc.) The conversion will still need to take place prior to offset printing, but chances are good that your printer will have color corrected monitors and an environment more suited to accurate color viewing. (Check with your printer first before submitting the files.)

Double Gatefold vs. Double Parallel Fold

Imagine four 8.5" x 11" panels side by side (for a flat size of 34" x 11"). Let's say this is a glossy brochure you're designing for a cosmetics client. You have at least three options for folding.

• For a double gatefold, you would fold the left and right outermost panels inward. This would yield a 17" x 11" piece, which you can then fold in half again for a final folded size of 8.5" x 11".
• For a double parallel fold, you would fold the left half of the 34" x 11" piece over the right half, yielding a 17" x 11" intermediate piece. Then, you would fold this in half again for a final folded size of 8.5" x 11".
• The third option would be to fold the 34" x 11" flat piece into a zig-zag pattern (with each panel folded in the opposite direction of the prior panel, like a series of attached "Z's"). This is called an accordion fold.

Why would you choose one folding pattern over another? It would depend entirely on how you want to lead the reader's eye through the brochure. Another way of saying this is that it would depend on how you want to organize the material in your brochure. This is a design decision.

• A double gatefold showcases the center panels. It also lets you hide them until the reader opens the two outer panels.
• A double parallel fold lets you lead the viewer from the front panel to a two-page intermediate inner-spread to a final, four-page-wide spread.
• And an accordion fold lets you display a full, four-panel spread on one side and then a full, four-panel spread on the reverse side of the brochure.

It all depends on how you want to present the content of your brochure.

[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]