There are two schools of thought when it comes to preparing files for printing, and both of those have to do with how the file is to be imaged. Let’s explore both options and why you need to save InDesign in either CMYK or RGB. As a bonus, I will toss in an added tip!
The reasons for CMYK
Traditional litho presses print in what is called four-color process, and those colors use the acronym CMYK, which stands for Cyan, Magenta, Yellow and Key, which is black. Each print unit on a press has the ink fountain, the rollers, the plate, the blanket and impression cylinder to image one color on the press sheet as it passes through the press. Subsequently, files are processed using a RIP device to yield that CMYK mix, so your printer prefers to have the image and/or linked files in CMYK, not RGB. If the files come in to them as RGB, they have to be converted to CMYK and that may change the color of the image, which means costly color correction.
The reasons for RGB
In digital print, the process is different. RGB has a bigger gamut of tonal range and therefore is easier for the digital device to image that file with a full range of color. If you leave the image in RGB, the RIP on the digital press can interpret that and make best use of the RGB color gamut, mapping out the color rangers more accurately into the digitally imaged CMYK gamut.
An added tip: what to do with PMS colors
This tip is for digital print, but can apply to litho as well. In digital press operations, all color is done in CMYK – there is rarely a “5th unit” that can print a PMS color. Granted, some of the newer digital presses have 5th units, but those are for special applications, such as clear toner or standard colors (blue, red, or even using it as a 2nd hit of a process color). But the best way to prepare a file for digital printing that has PMS colors in it is to leave those colors called out as PMS, do NOT convert them to CMYK. Digital presses have become so sophisticated in how they RIP PMS colors that they can get to very close approximations of those PMS colors. There are exceptions, and a good print provider will have a PMS swatch book that is printed direct from their device and shows how close the digital press can get to a specific PMS color. If it is critical that the PMS match, the printer can do color corrections on either the device or on the file, and run proofs, until the color comes close to what the client desires. However, the client must know that the device is limited to CMYK only, and that they may or may not reach the color that best matches a corporate color.
The key in all of this is to ask your print services provider how they prefer the files for the most optimal imaging or printing, be it litho or digital. Establishing a good line of communication with the actual department heads or production personnel can be of a great benefit as well, since they can be the ones that can collaborate with your design team to insure the files best meet their production standards.