Basic Color Theory for Printing

Photo. Magnet letters C, M, Y, and K are arranged on a desk with various office supplies.
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Light and ink mix colors differently, so how can the design on a computer screen match what is actually printed?

When all the colors of light mix together, the result is white. Light mixtures are additive. For example, computer screens use an additive mixture using Red, Green, and Blue (RGB). However, when all the colors of ink or paint mix together, the result is black (actually, brown). Instead, inks and paints are subtractive. Printers, for example, combine Cyan, Magenta, Yellow, and Black inks (CMYK). No printer can print in RGB.

Designing on a computer often must account for switching from a RGB workflow to CMYK to match the drafts to the printed outcomes.

RGB: an additive color mode using red, green, and blue light.

CMYK: a subtractive color using cyan, magenta, yellow, and black ink or toner.

RGB & CMYK Color Theory: How Printers Convert RGB to CMYK

First, a printer receives data from the computer in RGB mode. Then, the printer must convert it to CMYK. This means that computer screens are able to display a much wider range of colors than what printers can produce. Users can calibrate their monitors to match printed colors with downloadable ICC Profiles. ICC Profiles can be input, such as when scanning a photograph to a computer. ICC Profiles can also be output when sending a file from a computer to a printer, too. Some ICC profiles are standardized so multiple printers can match colors, but several manufacturers and printer models also use their own unique ICC Profile. Differing ICC Profiles can cause a design to look great when printed at home, but muddy when printed at a friend’s house.

Graphic. Converting from RGB on the computer screen to CMYK for the printer using ICC profiles.

Digital Printing

You are most familiar with Digital Printers, also known as “everyday printing.” Each digital printer uses its own RGB to CMYK conversion formula using a Raster Image Processor (RIP) to produce colors using toner rather than inks. Desktop printers; copy centers like Staples, Office Depot, and FedEx Kinko’s; and some print shops like VistaPrint use their own ICC Profiles to convert RGB images to CMYK for printing.

These digital printers:

  • are local, and easy to find and use at home.
  • can closely match CMYK outcomes to the original RGB image with RIPs.

However, digital printers might:

  • fail to match a design’s color and quality across multiple printers.
  • cost more to print in huge quantities.
A PDF proof of the poster from SPC's HERO The Rock Opera shown on a tablet.
PDF in RGB
A poster mock-up of SPC's HERO The Rock Opera.
Poster in CMYK
A mock-up of the screen-printed design of the logo from SPC's HERO The Rock Opera.
Screenprint in Spot

Professional Printing

To ensure designs look the same when printed anywhere and to even save money on large print jobs, there is Professional 4-Color Process Printing as well as Spot Printing. Process Printing uses standardized ICC Profiles for color matching, the most famous of which includes the Pantone (PMS) color system. Process printers separate the images into C, M, Y, and K layers, then overlap the inks to create a multitude of colors.

If printing in multiple colors is too costly or not an option—for example, screen-printing t-shirts—you can use a limited color palette of one to a few PMS colors, or even Spot Colors. In 4-Color printing, green is created by adding C and Y inks. In Spot Colors, green is just green. It is mixed before-hand, all in one batch, and nothing is mixed while actually printing. Spot Color printing creates solid colors and does not need halftone dotting to mix colors the way 4-Color Process Printing does.

How Designers Help The Conversion Process Using Color Theory Basics

Color theory basics makes obvious that RGB and CMYK color modes have different strengths and limitations. Graphic designers know to work in RGB for online images and in CMYK for printed images. However, CMYK has a more limited color space than RGB, making very bright, saturated colors difficult to reproduce in print. On the other hand, CMYK can produce more vivid blacks. Knowing the different color theory basics and needs of each color mode is important when beginning the Design Process. There is a lot of ongoing debate over whether to make printed images in CMYK from the start, or to create in RGB then convert at the project’s finish.

During the design process, a designer can utilize the greater color range of RGB and many of Photoshop’s RGB-only filters and effects. The designer must also plan ahead for the CMYK conversion later. This is where Soft-Proofing begins. Photoshop allows designers to create a Proof Setup in “Working CMYK,” then highlights “Out of Gamut” warnings of RGB colors that cannot be converted well to CMYK. Knowing the basics of color theory help prepare the design for a good conversion.

A combination of techniques creates an ideal compromise between the RGB workflow and CMYK soft-proofing for a more seamless conversion transition. Those techniques might include:

  • setting the color mode to “Working CMYK;”
  • making Selective Color Adjustments;
  • decreasing Vibrance and Saturation; or
  • painting over an “Out of Gamut” color range.
Screenshots. Regular viewing (left) with colors marked "out of gamut" in gray during soft-proofing (right).
A demonstration of a soft-proofing technique in Photoshop. Regular viewing (left) with colors marked “out of gamut” in gray during soft-proofing (right).

Matching The Screen To The Print

Sometimes the process from on-screen to on-paper presents challenges to the designer and clients. However, an understanding of color theory basics, the printing process, and soft-proofing techniques allows the designer to convert on-screen, RGB designs, into production-ready, 4-color prints successfully.