Each of the process colors is layed down on the page one at a time in varying amounts.
Learn how to specify and print process colors and browse a gallery of CMYK color palettes. Other related issues include Euroscale, SWOP, Trumatch, and Hexachrome colors.
It's imperative that you understand CMYK to do desktop publishing and commercial printing. It's explained and illustrated here.
Before You Buy PANTONE Swatch Books
This explains the types of swatch books available in general terms.
ColorMap - Printed Guide for CMYK (review)
This handy printed guide is like a mini-swatch book for CMYK colors. It also includes The Color Harmonizer, a type of color wheel with tips to help you pick pleasing color combinations. Nice, inexpensive item to add to your toolbox.
Modern and Historic Color Palettes
The color palettes here utilize CMYK color formulas.
PANTONE Process Color Guides
You can't trust your monitor or your desktop printer when it comes to accurate color-matching. Pantone guides are a far better way to choose colors for your printed projects. Find just the right color for the paper stock you'll be using. One of these swatch books are what you'll want to add to your toolbox for all your process (4-color/CMYK)...
Use Imposition to Save Money on Color Printing
Reduce the cost of four-color process printing.