What is Your Color Space? Part 3, CMYK.

Professional printers use CMYK (Cyan, Magenta, Yellow, Black) to print full-color text, graphics, and photos on just about anything. These colors are called Process Colors and, when printed, the inks are laid down light to dark. CMYK is known as a Subtractive Color Model because every time one ink overlays another the color gets darker and it subtracts from the reflectance of the substrate.

So, how does CMYK work?

Let’s take a picture of an old red barn surrounded by green grass, against a blue sky. First, that picture would be Color Separated into its CMYK components. Since this is a photo, the printer would use a Halftone Screen. This screen breaks the image up into little dots and controls the amount of ink being laid down in any particular area of the photo. In the separation (a negative image), the darker areas contain small dots, allowing more ink to be transferred to the paper by the plate. In lighter areas these dots are large, so the plate picks up and carries less ink to the paper, resulting in lighter printed tones.

Now we have our four plates and the press is inked up. We’re ready to print thousands of copies of our barn landscape. How do we get those brilliant reds, greens and blues? That’s easy (at least in theory). CMY are secondary colors in the RGB color model we spoke about last time. By mixing any two secondary colors you get one of the RGB primaries.

For the barn, the red will be a mixture of magenta and yellow. The green grass is a mixture of the yellow and blue and the blue sky is a mixture of magenta and cyan. (See photo)

Final Question: Why do we need a black (K) plate in four-color printing?
Answer: In theory, the secondary colors CMY, when printed at 100% saturation, should add up to a solid black. But that’s only in theory. The fact is that without the black added we end up with a dark muddy brown. This is not a problem with the theory, but instead it is the imperfections of the inks. Adding black is the only way a printer can get richness and depth in a printed piece.