RGB vs. CMYK

One problem many beginners in the prepress industry have concerns viewing CMYK on their RGB monitors. Even though programs such as Adobe PhotoShop "show" files in CMYK, they are surprised when these colors look different on either their color printers or on the press. Many programs attempt to approximate the CMYK values, but are unable to show the exact color, since monitors can only display in RGB and can only approximate colors in a different color model.

Another problem is using PhotoShop files in RGB or Indexed Color formats instead of CMYK when creating 4-Color files is that only devices such as film recorders (for 35mm slides, 4'x5" transparencies, etc.) can use RGB PhotoShop files. All other output devices (i.e., printers, poster printers, imagesetters) require your PhotoShop files to be Bitmaps, Halftones (Grayscale), Duotones, or CMYK files. Indexed Color (used primarily for the GIF format) should be reserved for display images only (web sites, on screen slide presentations for example), and not even considered for output.

Color Models

A color model is a mathematical way of describing color. The color model we use depends upon which form of light (emitted or reflected) that we are dealing with. The colors that we see and can differentiate between are merely different wavelengths of light. Physicists can describe five different ways in which we perceive light, but in prepress ( and postpress, too, for the matter) we rely on only two of them: emitted light and reflected light. This difference here is important because it determines how the lightwaves are manipulated in order to create the color we wish to see.

The RGB Color Model

When we deal with emitted light we use the RGB (Red, Green, Blue) color model. By varying the intensity of two primary colors (red and blue) and one secondary color (green), a TV set or color monitor can display more colors (over 16.7 million) than the human eye can distinguish between. The RGB color model is an additive model. What that means is that if 100% of all three colors are added together, the resulting color is pure white.

The CMYK Color Model

When we deal with reflected light we use either the CMYK or an alternative/complementary color model (such as the Pantone® Matching System. This is because paper, unlike a TV set or monitor, does not emit light. It can only reflect light! The inks used absorb certain wavelengths of the light bouncing off of them. For example, the ink that we call "yellow" absorbs every other wavelength of light except for the yellow portion of the light spectrum, which is, all that is reflected. As a result we only see the reflected wavelengths. The CMYK and alternative/complementary models are subtractive. If 100% of all colors involved are removed, the result is pure white (or whatever the color of the paper happens to be). The model is used for color printers as well as the printing press by combining different amounts of these four colors on paper or other stock/media.

The Pantone® Color Model

The Pantone Matching System is a color model similar to the CMYK model. It can be used in place of, or in addition to, the CMYK model. Unlike the RGB and CMYK models, the PMS system, does not try to create as many colors as possible, but instead, has a strict subset which can be found in their swatchbooks. The PMS system is created by combining not only various amounts of CMYK inks, but also "nonstandard" inks such as Rhodamine Red, Reflex Blue and Cool Gray. These are base inks in the Pantone System which are combined with each other, as well as with cyan, magenta, yellow and black, to create other colors. Reflex Blue, for example, is made from a pure pigment (spot or solid color) and is not created by combining different amounts of cyan, magenta, yellow, or black. Because of the tremendous growth of the prepress industry and the dependence of nearly all output programs on the CMYK color model, the Pantone Corporation developed CMYK equivalents to many of its ink-based colors. Even though the CMYK equivalents are considered to be "close" in color, they still do not match precisely. By comparing true Reflex Blue and its CMYK equivalent (C=100, M=72, Y=0, K=6), one can see that the two colors do not match, however, Pantone Corporation has determined that this equivalent is the closest that they could get to Reflex Blue by using the four base inks of the CMYK color model.

The bottom line is that there is no substitution for careful consideration of colors to be used in developing a project for printing. Even designers with many years of experience with the four-color printing process must remain very aware of the colors that are reproducible using the CMYK color model. It is my sincere hope that the information contained above has broadened your understanding of the differences between color models. Perhaps now you will be able to give a more precise answer to the question "Why Does It Look Different On My Monitor?".