RGB, CMYK (also called PROCESS), SPOT, INDEX, GRAYSCALE
If you have been thrust into the world of desktop publishing simply because your computer can make pretty documents, you will be able to get the output you want if you understand how the different color options are defined and when to use them.
RGB = Red, Green, Blue
CMYK = Cyan, Magenta, Yellow, Black
SPOT = numerous options here, most popular with printing is the Pantone Matching System (PMS)
INDEX = mostly associated with GIF files
GRAYSCALE = as the name implies, shades of black 100% to 0%
PC's do most of their work in RGB in an office environment. (Word, Excel, Powerpoint, Web Browsers--RGB files work fine.) Desktop printers can print RGB files with little or no surprises.
If you take your documents to a commercial printer, they will want files in CMYK, SPOT or GRAYSCALE. Converting RGB to CMYK will most likely change the way some of the colors look. That's just the way it is unless someone REALLY skilled does the color conversion.
SPOT colors are the most difficult to create when using standard desktop publishing software, especially if you mix bitmapped files with vector files created in different programs, which they most likely will be. (see previous tip about file formats). If you must print in spot colors, make sure the printing firm you choose has the correct software to redefine the colors as needed.
If you are creating something that is a single spot color, you can actually create it in grayscale and it will work just fine at a commercial printing firm.
Web colors have so many different types of computers and displays that interpret the intended color, it is almost impossible to get consistency. That's why the web safe colors were defined. Monitor adjustments are still subjective so even then, the limited palette you use is not a guarantee match. For the most part, a well compressed JPG will work fine.
--KH