Shannon Noack has published an article on how to use color theory in web design. After a quick review of basic terminology, she takes a look at some well known sites to see what colors they used, and what that means to viewers of the site. She takes a look at the White House web site, Amazon, Best Buy, Verizon, and a few others. Overall it is a pretty good read. From the article:

Unarguably one of the most important aspects of any design is its colors. Designers create the style of a site, as well as the movement it makes, the emotion it creates, and its purpose based largely upon the color choices they make. Colors are powerful tools and an important thing all designers should understand when creating websites.

via A Look into Color Theory in Web Design.