Converting color images into files that can be used on a printing press is very, very complicated. Computers and printing presses use two different color systems: computers use RGB and presses use CMYK.

And RGB has to be converted to CMYK before a job prints.

RGB stands for red, green, and blue. Color on a computer monitor consists of millions of dots in these three colors.

CMYK refers to the four colors used for full-color process printing:
C for cyan (blue), M for magenta (red), Y for yellow, and K for black.
Full-color images in printing, like photos and illustrations, consist of patterns of these four colors.

“Who should convert the files the designer or the printer?” I was asked recently. I checked with trusted colleagues, both designers and printers. The answer? “It depends.”

Whoever does the RGB-to-CMYK conversion has more control over how the color separations are built.

Designers experienced with printing may prefer to convert RGB to CMYK, using different image-processing software. According to my printer-source, this isn’t like saving a file with a different name. It’s more like telling a software program to build color separations, and this isn’t a straightforward process. Sometimes designers don’t know exactly how their files will be converted so they won’t have any idea what their images will look like in print until they see a proof, which is far too late in the process.

Some designers send in RGB files, knowing that their printer will do the conversion expertly. Other designers say that it depends on the images and where they came from. Sometimes they bring images into their designs that have already been separated into CMYK.

Keep in mind that colors are device-dependent, so what you see on your monitor differs from what I might see on mine and so on and so on.

Frank Romano sums it all up in his Pocket Guide to Digital Prepress, where he wrote: “What the scanner sees, the monitor shows, the proofer proofs, the film images, and the press prints are essentially different.”

If you’re a designer, call your printer to discuss RGB-to-CMYK conversion. If you’re a printer, let your customers know the pitfalls.