Color Management Primer – RGB vs. CMYK

 Color management, if followed correctly, ensures color dependability throughout a digital process. To get the best results, you need to understand some basic principles.

RGB does not match up to CMYK

In today’s digital workflow, we use a wide variety of image capturing devices and an equally large number of output devices to create prints. Due to a variety of factors, a digital file cannot perfectly correspond to a printed sheet. The most evident mismatch is because we use Red, Green, and Blue (RGB) to describe digital image colors, but we print with Cyan, Magenta, Yellow, and Black ink (CMYK). Color management helps to minimize these factors so that a consistent and fairly accurate color match is possible.

RGB

RGB as defined by Wikipedia

The RGB color model is an additive color model in which redgreen, and blue light is added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.

The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography. Before the electronic age, the RGB color model already had a solid theory behind it, based in human perception of colors.

RGB is a device-dependent color model: different devices detect or reproduce a given RGB value differently, since the color elements (such as phosphors or dyes) and their response to the individual R, G, and B levels vary from manufacturer to manufacturer, or even in the same device over time. Thus an RGB value does not define the same color across devices without some kind of color management.

Typical RGB input devices are color TV and video camerasimage scanners, and digital cameras. Typical RGB output devices are TV sets of various technologies (CRTLCDplasma, etc.), computer and mobile phone displays, video projectors, multicolor LED displays, and large screens such as JumboTron, etc. Color printers, on the other hand, are not RGB devices, but subtractive color devices (typically CMYK color model).

CMYK

 

CMYK as defined by Wikipedia

The CMYK color model (process colorfour color) is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing:cyanmagentayellow, and key (black). Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the abbreviation.

The “K” in CMYK stands for key since in four-color printing cyan, magenta, and yellow printing plates are carefully keyed or aligned with the key of the black key plate. Some sources suggest that the “K” in CMYK comes from the last letter in “black” and was chosen because B already means blue.[1][2] However, this explanation, though plausible and useful as a mnemonic, is incorrect.[3]

The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called subtractive because inks “subtract” brightness from white.

RGB to CMYK Conversion in Printing

When a digital image is printed, its RGB numbers are converted to CMYK numbers for the printer. This conversion will produce unexpected color if not done in a controlled and predictable manner. Overlay RGB and CMYK color spaces and you’ll see that colors in RGB that do not have an exact equivalent in CMYK. What this means is that there are often colors that we see on our monitors that cannot be reproduced perfectly on the printed sheet. Color management is a process by which we pick the best possible CMYK color to match a given RGB color.

Color Profiling For Consistent Color

With all of these conversions from one color space to another, from one device to another, the color of an image would appear differently on each. What a color profile does is describe what the colors will look like on a particular device whether it is a computer monitor, an ink-jet printer, or a digital press. The color profile contains a Look-Up Table it uses when fed the data that describes a certain color on your monitor and converts it to the same color on a digital press.

In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards disseminated by the International Color Consortium (ICC).

The standard ICC Profile is based on the GRACoL2009 reference used in high-end commercial printing. The entire print network adheres to this standard on all of their print devices for the most consistent results possible with print on demand. By using this color profile, you may soft proof your images while in RGB to see how they will look when printed, or use the profile to actually convert your images to the CMYK color space of the print device to eliminate the press-side conversion. This gives you more control over the images and how they will eventually print.

 

by Rob Zelinsky,
Digital Operations Manager,
AlphaGraphics in the Cultural District

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