Illegal Colors
http://www.graphic-design.com/DTG/Graphics/IlColors.html

Cohen, Sandee
Design, Typography and Graphics

Abstract:

Illegal colors are those colors which you can pick on your computer that will not reproduce using traditional CMYK offset printing. For example, open the Apple color wheel. Set the brightness as high as it can be and click on the aqua color at nine o'clock on the wheel. You see that bright, glowy, neon-like color? Isn't it beautiful? Too bad, it's illegal. There is no way that ordinary CMYK inks can reproduce that color. Most people get into illegal color trouble when they pick a color that looks great on the screen, and then have the job printed using process colors. They then complain to the print shop that the job looks dull, that all the brightness is gone. That's the fault of picking illegal colors. You won't get arrested but you will be disappointed in the output. The following is a list of some of the popular desktop publishing program and how they handle illegal colors.