Montana State University

About Color

Basic Color Theory


There are several different color models used in the various reproduction methods for printing and computer monitor design. Each of these is intended to achieve optimal quality for different intended uses. It is useful to have a basic knowledge of these models in order to make the best color choices for your project. The three basic categories of color that are most often used in graphic production are RGB, CMYK and Spot color. Each of these is discussed below. However, in order to better understand these differences, a little color theory is helpful.

One of the most important concepts for understanding color models is idea of color gamut. The entire range of the color spectrum is extremely broad, extending beyond the range of human visual perception, from infrared to ultraviolet. There are millions of colors that can exist in this range. However, not all of these colors can actually be created or displayed by devices such as computer monitors, scanners, digital cameras, printers, photographic film, printing presses, etc. These devices are only capable of using a limited subset of the full spectrum and each generally uses a different subset. Color gamut is the range of color that can actually exist within any color model, for any particular device. Subsets of the overall color spectrum are also sometimes referred to as a color space.

The problem frequently becomes one of moving an image through a production workflow where every device has a slightly different color gamut. Imagine scanning into one color space, then editing the image on your computer monitor in another slightly different color space, printing to a desktop color printer with another color space and then finally outputting to film for offset press in yet another color space. The process of dealing with these issues and mitigating the differences is known as color management.

Although there may be large areas of the gamut that overlap between devices, there are also areas that do not. The classic example of this is the difference is between pigment-based colors used in printing, and light-based colors used in computer monitors and televisions. These are often called additive and subtractive color. In additive color white is created by combining all colors of light, such as on a computer monitor. In subtractive color, white is the absence of all color pigment, such as on a printed page. Due to radical differences between these color spaces, it is very easy to create colors on your monitor that cannot be reproduced in print. This is one reason why you should never completely trust what you see on screen as a reliable indicator of what will print.

Color management can be a very challenging when dealing with rigid color specifications. Knowledgeable designers and graphic producers know to deal with these issues to minimize problems and maximize predictability. However, some projects are complex enough to where absolute predictability is not possible. This is why designers frequently remind clients that color matching of the materials they are viewing may be imprecise.

As stated previously, there are three basic color models that are most useful for graphic design projects. In some software applications, such as Photoshop, these are also referred to as color modes.

RGB Color


The first of these is RGB color - or Red, Green, Blue. This is a subtractive color system based on combining light to produce color. The basic idea is that beams of light in each of the three colors, R-G-B, can be combined in various intensities to create a wide range of other colors. This is how televisions, computer monitors, digital cameras, and low-end scanners produce color -- for the obvious reason that these are all light-based devices. Since computer monitors are most often the first step in digital imaging, RGB is also the default color space for many software applications.

The variations in the intensities of the R-G-B components can be defined mathematically by assigning a range of values from highest to lowest intensity for each. In this way specific colors can be recreated by using specific values. In most RGB color systems these values range from 0 to 255 for each component. For example, in the RGB color picker of an application such as Photoshop 0--0--0 is black and 255--255--255 is white. All intermediate colors fall in between. The RGB values for the nearest color to MSU blue is: R=12, G=45 and B=131 or 12--45--131. For MSU gold the nearest value is R=255, G=186 and B=0 or 255--186--0.

RGB is generally the final destination color mode for anything that has a computer monitor as its primary display, such as a Web page. Although these images do print to color printers, they are not expected to be color accurate and will often exhibit color shifts when printed. (This is because most color printers operate in a different color mode using process colors, or an additive color system. See the next section.) For projects intended to end in print, RGB is generally only an interim mode used as a working space, but ultimately intended for conversion to CMYK.

Even within RGB there are several variations that each have different color gamut. Image editing applications such as Photoshop allow you to select among these to optimize your results. Two of most useful are AdobeRGB and sRGB. AdobeRGB is a wide-gamut color mode that has been optimized to be more compatible with printing projects and conversion to other modes. If your project is intended for print, and you need to use RGB images in your workflow, AdobeRGB is a highly recommended option. sRGB is a narrower gamut mode that is optimized for images that appear on computer monitors. Microsoft uses sRGB as the default color mode for displays within its operating systems since it represents the color space of an "average Windows monitor." Although it may be more limited in gamut, working in a color mode that is indexed to the greatest number of users can be advantageous for achieving predictable results. sRGB has also gained favor as a proposed standard for an Internet color space.

Another variation of RGB is called Indexed Color. This is still an RGB mode, but instead of being a full range of colors defined by the range of 0--0--0 to 255--255--255, there are a much more limited number of colors allowed in the image. A pre-defined color table included as part of the file controls the range of colors available and every color used in the image is "indexed" to this table. Color variation can be achieved by dithering, but every pixel must match a color in the table. Color tables can be set to different sizes and specific colors, providing a way to optimize images.

One of the common color tables is called the "Web-safe" palette. Both PCs and Macs allow 256 colors in their operating system for 8-bit display settings. This is a common display setting for monitors. However, there are only 216 colors that match between the two systems. The Web-safe palette indexes images to these common colors so that colors will be more consistent across platforms. These Web-safe colors also have reference codes referred to as hexadecimal numbers. These codes can be used in building Web pages to assign specific colors to components of the Web page. For instance, the hexadecimal code for black is "#000000" and the code for white is "#FFFFFF". The nearest hexadecimal color for MSU blue is "#003399" and for MSU gold is "#FFCC00".

File formats such as the popular CompuServe GIF use this indexing technique. The advantage is that by reducing the number of colors to a limited range, file sizes can be greatly decreased through more efficient file compression. This is highly desirable for the Web. The disadvantage is that the limited set of colors does not provide the best quality display for many types of images.

CMYK -- Process Color


The next important color mode is known as CMYK or Process Color. This is an additive process that is based on combining pigments in varying amount to produce color. These pigments have been standardized into four colors: Cyan (C), Magenta (M), Yellow (Y) and Black (K). The standardization has become known as the process -- thus process color. Specific colors in this system are always cited as percentages of the four component parts in the C-M-Y-K order. For instance, the process color formula for MSU Blue is: C=100%, M=69%, Y=0% and K=11% or 100--69--0--11. For MSU gold the formula is C=0%, M=27%, Y=100% and K=0% or 0--27--100--0. The CMYK formula for white has all four values of 0%. Black has all values of 0% except K, which is 100%.

Process color is the standard color mode for printing production. Everything from low-end desktop inkjet printers to high-end offset presses the size of a Greyhound bus use this system to create full-color reproduction. CMYK is generally always the best color destination space for printed projects.

As with RGB, there are numerous ways to configure CMYK files for different printing methods. However, with process color the differences are less about gamut and more about the mechanics of printing. The key to achieving the best reproduction within this space is making sure that the attributes of your file are set up to accommodate these differences. These include technical considerations such as dot-gain and under-color removal, to name a few. Taking control of, and responsibility for these can be complex. The assistance of an experienced professional is highly recommended where high-quality color standards are required.

As noted previously, RGB files will usually output to CMYK color devices. But there is always a color conversion performed by the device, and this conversion may not be an accurate one. It is generally better to do your own conversion and take control of the details. This is why many print suppliers require that all files be in CMYK mode.

Spot Color -- Pantone Colors


Spot color is another additive process color model where color is achieved using the pigment of inks, either singularly or in combination. However, unlike process color, the inks are not limited to four base colors. An entire range of different inks can be mixed. The most common application of this system is through the Pantone Matching System, or PMS colors. The key to this system is a printed color reference guide where all the color inks are displayed. The user can select a color from the guide that can be reliably matched by simply mixing that ink according to a formula.

There are hundreds of these colors to choose from, but to preview these accurately you must have access to a printed reference guide. Unfortunately, these are somewhat expensive, and different guides are required for different papers, such as coated and uncoated. This is due to the fact that different papers absorb ink differently and thus alter the color. The color of different papers will also alter the perception of the colors.

The primary application of spot colors is on projects intended for print on an offset press. This system lends itself well to providing smooth color solids and pleasing variations through the use of tints. Another advantage is that specific color matches can be created very reliably. This is often important when dealing with important identity elements such as logos, where precise color matching is important. For instance, the official Pantone color for MSU blue is PMS 287. For MSU gold, PMS 129 is used when printing on uncoated papers and PMS 130 is used when printing on coated papers.

One disadvantage is that with so many ink variables, there is no standardized process for creating photo-realistic color such as there is with CMYK. If you need true color images, you must use CMYK inks in addition to spot color inks. This is much more expensive. Another problem is matching spot colors to process color equivalents. Due to the limitations of gamut for CMYK, not all spot colors can be precisely matched in process. In some cases the differences are significant. The specifications for the nearest process color equivalents of the MSU colors are given in the process color section above.

About Color Photographic Quality


The technical issues related to achieving high-quality color for photographic images for print are certainly not trivial. There are numerous variables that will affect the final result. Many of these are determined by decisions that are made at the very outset of image selection and production. Unless you have gone to a great deal of expense and trouble to carefully calibrate your entire image editing workflow -- from image scanning to color proofing -- the surest rule in this business is what-you-see will NOT be what-you-get.

Professional service providers in this field have access to much higher quality equipment for image acquisition and editing. In addition, they have experience at dealing with issues such as cast removal, color correction to specific devices, digital spotting and image editing, selective sharpening, dot gain compensation, and several other issues important where the highest quality is required.

If predictable, quality color is of paramount importance for your project, you should consult with our staff about your requirements. We are experts at producing projects to the highest color standards and can assist you with this process. High quality color reproduction doesn't have to be expensive, but it does have to be planned and executed well.

Written August 2001.