Full Color Printing – CMYK Separation

What is CMYK?

CMYK is an acronym for Cyan, Magenta, Yellow & Key (black). CMYK can also be referred to as process color, full color or four color printing. Using different combinations of CMYK, this color model can produce a full range of colors which gives both digital and offset printers the power to reproduce both colored text and images with astonishing results.When an image is printed, it is really a collection of tiny dots of colors that make up the colors of the image. This is similar to how digital images are displayed in pixels. If an image on the web has a low PPI (Pixels per Inch) it will become grainy if you try to make it too large.

How Does CMYK work?

CMYK is a subtractive color model meaning that color is added to a white background to subtract the amount of white space. Cyan, Magenta, Yellow and Black inks are applied to a sheet of paper (or other medium) to make up an image out of all the possible color combinations. An image is separated into dots, which are tiny color selections to make up an image. The amount of dots per inch make up the resolution of the image, and having a dpi of 300 or more will print a vibrant image with smooth and clear lines.

The image to the right demonstrates how an image is split into four color plates using the CMYK colors to make up the full color image on the top.

Separating the Colors

The set of images to the right is a visual description of the separation of colors in an image. The process is actually separating an image into dots for each color (Cyan, Magenta, Yellow, Black). This setup mimics a smooth, continuous color tone to the human eye, instead of being pixilated or blotchy. These dots are measured in Dots Per Inch (DPI), which measures the resolution of an image.

If you look even closer, you would notice that each dot is not 100% solid color; it is actually a transparency and is overlaid by the next color in the sequence (CMYK) to form a new color. This explains how to build a a color in CMYK format. You select a percent of each CMYK to make a blend of the actual color that you want. For example, the image below contains CMYK values of (71, 18, 47, 16) respectively to make up this unique shade of blue.

Separation of a picture of tree and sky into cyan, magenta, yellow and black | mmprint.com

Separating the Colors

The image below is a printed swatch of the blue color above which is magnified under a device called a loupe. In it, you can see how the color is broken up into tiny dots of different colors, even though it appears as one solid, smooth color to a normal eye.

Printing creates colors by combining different levels of cyan magenta yellow and black | mmprint.com
zoomed in picture of a printed color swatch showing the different arrangements of dots to create a solid color | mmprint.com

How is artwork converted to CMYK?

The conversion of artwork into CMYK is called color separation. When you are setting up a document for print, it should always be setup in CYMK (or Pantone spot colors for if needed). Digital images that are taken from cameras or scanned onto a computer generally are made up of pixels which contain the RGB (red, green, blue) color system. These images must be transformed from RGB to CMYK for printing purposes. Design programs such as Adobe Photoshop and Illustrator, and Microsoft Publisher can convert colors with little or no effect to the overall image colors.

The process of converting RGB colors to CMYK involves color separation of RGB colors and inverting them to their corresponding CMYK colors.  Black ink is also used to add shades and darkness to an image.

What About the Black Ink?

It is cost effective to use 100% black ink when printing a document with no colors. As mentioned above, this will only use black ink to print instead of a combination of CMYK which would use more ink and require more setup. If you are only printing in black ink, your order would be considered 1 color, or 1/1 instead of 4 color or 4/4.

Another advantage of printing with 100% black ink is the effect of plate registration. When the four CMYK colors are printed, the plates have to be calibrated so that each color plate prints on the exact same dot as the other colors. If the plates are misaligned by even a fraction of a centimeter, it can cause the color plates to print in slightly varied position. For a normal image, this would not be an issue, but for small black text, it can produce an image that is fuzzy and harder to read. This makes it important to set black colors on your file to CMYK value of 0, 0, 0, 100 (100% black).