Color Models...

Scientists have been theorizing and developing color models for years in the hope of coming up with an ideal way to describe color. These color models are nothing more than mathematical formulas for describing color. Modern day computers, operating systems and applications like Adobe Photoshop have no problem interpreting and converting from one color model to another. How do we quantify the visual aspects of color numerically or on a computer?

There are a number of different color models that describe and quantify color for us. There are device dependent color spaces like RGB (red, green, blue), which is the color model utilized by the monitor you are viewing this from and CMYK (cyan, magenta, yellow and black) the model used for ink on paper or other materials. Another model quite popular with graphics and imaging software applications is HSB (hue, saturation and brightness). We will take a look at all three of these color models and also one device independent color model CIELAB.

RGB Color Model

Anything that transmits, displays or captures light utilizes the RGB color model. Computers, scanners, televisions, digital cameras and human vision are all based on this color model. The three primary colors, red, green and blue constitute the RGB additive color model. As you add light the color becomes brighter, add red, green and blue light together in equal proportions and you will get white light. Different combinations of red, green and blue produce a myriad of different colors. If you work with 8 bit images in Adobe Photoshop you will have access to approximately 16.7 million colors and billions of colors at higher bit depths.

CMYK Color Model

The CMYK color model is based on ink on paper. These inks absorb and reflect different wavelengths of light. Cyan absorbs all red light, magenta absorbs all green light and yellow absorbs all blue light. If these three inks absorb all red, green and blue light then in theory the combination of all three on paper should produce black, in reality however there are no completely pure inks and as such not all of the red, green and blue light will be absorbed. The printing industry compensates for this problem by adding a fourth ink, black to the process. The addition of black allows us to produce good blacks on press and other output devices like inkjet printers.

The CMYK color model is called the subtractive color model because as additional inks are added color is subtracted, opposite of the RGB additive color model. Please note that RGB and CMY colors are opposite or complimentary colors. See the diagrams above and below.

HSB Color Model

This color model is based on Hue, Saturation and Brightness (or lightness). Hue is basically your color control; a hue can be red, orange, purple, green, etc. Saturation is the variable for the intensity or saturation of the color; how vivid a red or purple do you want? Brightness quite obviously controls the brightness or lightness of your color. The HSB model is still used extensively in image editing applications like Photoshop, in graphic design and is still popular with interior decorators for paint selection and categorization.

CIELAB Color Model

Last but not least is the CIELAB color model. Created in 1976 this is a refined color model based on the original CIE XYZ model developed in 1931 by the International Committee on Illumination or la Commission Internationale de l' Eclairage, the French translation.

CIELAB is based on a three dimensional model of color that includes the three primaries, L*, a* and b* (pronounced, L star, a star and b star).

As you look at this diagram, a* represents how green or red the color is, b* how yellow or blue and L* lightness. Although it may not be very intuitive to work with CIELAB it is a highly accurate mathematical model that emulates normal human color vision based on standard viewing conditions, light sources and a defined "standard observer" set by the CIE. This color model includes all of the colors visible to the human eye.

Device Independent

Unlike the previous color models mentioned here, CIELAB, because it is based on human vision and set standards is device independent. It is utilized in the color management workflow as a universal translator that enables us to move between color spaces without bias. For example if we started out with a digitally captured image in RGB mode we could use CIELAB and a conversion engine like Colorsync to convert the original RGB color values to CMYK for output to a printer with minimal change to the original color values.

CIELAB is a key component of the color managed workflow and without it managed color between devices would be extremely difficult if not impossible. You will most likely utilize all of the color models discussed here, get to know them all and the flow of color through your workflow will be easier to understand and manage.

Ok, let's move on to the next step... putting this all to together into a managed ICC color workflow.

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