|Red, Green, and Blue are the "additive colors " - combine red, green and blue light, and you get white light. Cyan, Magenta and Yellow are "subtractive colors" - if you print cyan, magenta and yellow inks on paper, they ought to absorb all the light shown on them. Your eye receives no reflected light from the paper, and perceives black... in theory.
In practice, printing inks contain impurities that prevent them from absorbing light perfectly. They do a pretty good job with light colors, but when you add them all together, they produce a murky brown rather than black. In order to get decent dark colors, black ink is added in increasing proportions, as the color gets darker and darker.
An image that is in RGB mode is optimized for display on a computer monitor. In order to reproduce that very same image using ink on paper, it must be converted to the "CMYK" color mode.
The word "gamut" is used to describe a range of reproducible color with a given set of tools. There are some colors which a computer monitor can display which are impossible to print using the standard "SWOP CMYK" inks in use across the United States (SWOP: standard web offset press). Most notably, certain vibrant deep blues and rich reds are "outside the gamut" of SWOP CMYK.
|In order to print properly, any image files that you supply for CMYK printing must be in CMYK mode [note that disc art is rarely CMYK]. RGB files will look good on screen, and they will even look good when printed on many of the desktop color printers on the market today. However, they will not separate properly when made into film, and the resulting full printing job will not look the way you expect it to look.
Inexperienced graphic designers, unfamiliar with the limitations of the SWOP CMYK gamut, supply us with a steady stream of RGB files, which we relentlessly convert to CMYK mode before sending for film output. Much of the time, the color change that occurs is slight. Every once in a while, though, we get artwork whose effectiveness is severely compromised when the color range is compressed during the transition to CMYK mode. It is often a challenging task to explain to the designer why there is absolutely no way to get that blue using CMYK, no matter how much we want to.
Recommended Workflow for designing for CMYK printing
Here are a few application specific tips.
Photoshop: Your scanner almost certainly generates RGB information. Don't worry, that's how it's supposed to work. In fact, you should leave your color files in RGB mode up until you need to print separations, or until you need to know CMYK ink values, so you can match colors in another program. While you're working, check how your files are going to look by turning on the "CMYK preview" mode. Don't make repeated changes between RGB and CMYK mode, using the mode menu. Every time you switch, a little clarity is lost. One switch is no problem; 20 switches makes a difference.
You may ask, why not simply switch to CMYK mode as soon as possible? 1) RGB files are 25% smaller, and are therefore 25% faster to work with and easier to store. 2) The SWOP CMYK gamut is pretty small. If you ever want to reproduce those files for a different medium (such as the web), you'll have thrown away some potentially useful information 3) Some filters only work in RGB mode.
Illustrator: Stick to CMYK and Greyscale color models. If you use Pantone Coated colors, make sure that 1) you convert them into CMYK mode or 2) you leave them as spot colors because you want to print spot color inks (and you're willing to pay all the associated upcharges). Stay away from RGB.
Quark Xpress: Familiarize yourself with the "Edit Colors" dialog box. Use only CMYK model and Pantone Coated model ink definitions. Make sure that you are in control of which colors should separate into CMYK and which should print as spot colors. Be aware that Quark's ability to represent color accurately is, shall we say, less than ideal.