Color differences between Computer Displays and Color Printers

Printed colors and colors as shown on computer displays are by nature very different. Light is emitted by a computer display at your eyes and the more of a given color (in each of the three red, green, and blue (RGB) channels) that is emitted, the stronger you see that color. Thus you see the strongest color, white, when the maximum amount of each of red, green and blue are emitted. As such, computer display colors are referred to as being additive (by sending a stronger signal, a monitor increases the amount of color/brightness). On a computer display, black is the background color and you see it when none of red, green or blue is emitted. On the other hand, when you look at colors on a page, the ink on the page absorbs the light and thus to see pure black, which is an absence of color, you must print out the maximum quantity of ink in each channel (channels with printers are cyan, magenta and yellow and often a pure black is also available (CMYK)). Thus, printer colors are reffered to as being subtractive (by outputting more ink, the printer reduces the amount of color/brightness you see). White is the color of the paper so nothing is printed to get white.

Because of this huge difference in the nature of how colors are seen on a computer display and on paper, there is an unavoidable difficulty in printing an image and having it look the same on the paper as it does on your display. Modern printers generally do an excellent job in minimizing the difference between displayed and printed colors but it is still well worth understanding the issues and why this match will basically never be perfect. The following image illustrates this problem:
Color Gamut The largest area in this figure is the CIE chromaticity diagram and basically includes all colors that can possibly be seen. The triangle inside this figure (points R-G-B connected with solid lines) approximates the number of colors which a present day computer display can show. The oddly shaped polygon which includes the points C-M-Y and is connected with dashed lines approximates the colors which a good color printer can output. These two figures are not only not the same but, in fact, neither is even a subset of the other. Printers and computer displays can each display certain colors that the other can not. As such, converting between the two color schemes is not totally possible. Approximations and compromises must be made and different methods for doing this are appropriate for different images.

Note Color gamut illustration above is from: Jackson, Richard, MacDonald, Lindsay and Freeman, Ken, Computer Generated Color, John Wiley & Sons, Ltd, West Sussex, England 1994, page 221