Color Notes

Some Basics
Additive vs. Subtractive
RGB uses additive color mixing, because it describes what kind of light needs to be emitted to produce a given color. Light is added together to create form from out of the darkness. RGB stores individual values for red, green and blue. RGBA is RGB with an additional channel, alpha, to indicate transparency.

CMYK uses subtractive color mixing used in the printing process, because it describes what kind of inks need to be applied so the light reflected from the substrate and through the inks produces a given color. One starts with a white substrate (canvas, page, etc.), and uses ink to subtract color from white to create an image. CMYK stores ink values for cyan, magenta, yellow and black. There are many CMYK color spaces for different sets of inks, substrates, and press characteristics (which change the dot gain or transfer function for each ink and thus change the appearance).

Gamut-Gamut is the term for the range of colors that can be represented in a given color model (RGB, CMYK, etc.) or a given output device (computer monitor, desktop printer, 4 color process, etc.). In Adobe programs, you can see when a selected color is out of gamut by the little triangle shaped warning sign that appears next to the color.

Color Space-The gamut, or available color range, of a device is called the "color space" of that device.

Color Profiles-The mathematical description of a device's color space is called the "color profile."

Color Management- A color management system (such as Adobe's) acts as the color interpreter, and uses the color profile of a given output device (printer) to keep your colors looking accurate.

In Illustrator, most color management controls can be found under Edit>Color Settings. When you place your mouse over a setting, you'll get a handy dialog box telling you the best circumstances to use various color profiles and management policies.

In light of the new capabilities of digital imaging it is recommended that you work in the RGB color space. Keep an original archive of the image in RGB format for additional jobs run on different output devices. Once a file is saved in a smaller color space, such as CMYK, the colors outside of that space are lost. Resaving the file in RGB will not replace the missing colors. Let your service provider modify the color space to best suit your output needs. It’s a good idea to contact your service provider and discuss color issues if you have any questions concerning color space requirements.

Here's a handy site with some useful printing tips: http://www.printernational.org/