LR Tips CMYK vs. RGB


Senior Member
Sep 27, 2006
Claudia McCue


Excerpted from Real World Print Production with Adobe Creative Cloud by Claudia McCue. 
Copyright © 2014. Used with permission of Pearson Education, Inc. and Peachpit Press.
* * *​
Since the dawn of desktop publishing, it’s been unquestioned that Thou Shalt Convert to CMYK. Those who submitted RGB files were considered uninformed, even uncivilized.
The rules are changing, though, because of the increased use of digital printing. Although these devices may use inks or toners named cyan, magenta, yellow, and black, those inks and toners have a different pigment makeup than the namesake inks used on offset presses, and they have a wider color gamut than offset inks. Inkjet devices such as large-format printers utilize additional inks such as light cyan, pink, light yellow, orange, and green, further extending the range of colors that they can print.
This seems like a good time to open a can of multicolored worms. After you’ve been told by printers for years that you should convert your images to CMYK before submitting, I’m now going to tell you that you might not have to do so. That’s because many digital devices happily digest RGB and can provide more vibrant output by rendering RGB content.
When you convert to CMYK, ranges of colors outside the CMYK gamut are remapped to fall within the CMYK printable gamut, and some of your most vibrant colors are lost forever.
If you happen to have some very colorful RGB images (tropical birds would do the trick), try this little experiment:
1. Open the RGB image in Photoshop, and maybe make it even more vibrant by using Hue/Saturation or Vibrance. Get carried away; this is for science, after all, not for art.
2. Choose Edit > Color Settings. At the top of the dialog box, choose North America Prepress 2 from the menu and click OK.
3. Choose View > Proof Colors. The difference in appearance may not be huge, but try toggling Proof Colors on and off quickly by using the keyboard shortcut (PC: Ctrl-Y; Mac: Cmd-Y) and watch for differences in bright blues and greens. Neon greens provide a particularly noticeable difference.
4. Choose View > Gamut Warning. The gray areas are areas whose current RGB color will be remapped (and probably become duller) when converted to CMYK, because of the smaller color gamut of CMYK.
This gives you an idea of the color range that you’ll lose when you convert to CMYK—and much of that color range can be imaged on many digital devices. Of course, ask the print service provider before you submit your work to ensure that you’re sending what they want. Just don’t be surprised if they say “RGB is OK.”
[h=3]RGB as a Working Format[/h]Because the RGB gamut is larger than that of CMYK, it’s often preferable to perform color corrections and compositing with RGB files, converting to CMYK (if necessary) as late in the process as possible. If you are participating in a fully color-managed workflow, you will keep your images as RGB with ICC profiles. The International Color Consortium (ICC) was formed by a group of graphic arts industry vendors, with the goal of promoting the use and standardization of color management tools. ICC profiles are methods of describing the characteristics of devices such as scanners, presses, and printers for optimal results. Conversion will not take place until the job is imaged. Much of today’s software offers sophisticated support of color management. For example, when exporting a PDF or printing, InDesign will perform the same conversion of RGB to CMYK that Photoshop would (assuming you’ve synchronized your color settings across all your Creative Cloud applications).
[h=3]What if the Printer Demands CMYK Images?[/h]Some print service providers and their customers have fully adopted colormanaged workflows as part of their regular operation. But many print service providers (especially in North America) expect CMYK when you submit your job, believing that it’s what Nature intended, especially when the job will be printed on an offset press (as opposed to a digital printer). Consult with your printer to see what they prefer. If you’re using digital photography or scanning your own artwork, they should be able to provide you with their preferred settings, so you can make appropriate conversions to CMYK.

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