The Art and Zen of Web Sites ( http://www.tlc-systems.com/webtips.shtml )
The standard web interface uses this principle by displaying the hypertext links to other pages in colors that stand out from the rest of the text. In addition, different colors are used to show which links have been visited and which haven't.
So strong are these clues, that you can look at a web page and tell a lot about it without reading the text.
It's also a good reason for leaving the text and links in their default colors. Your visitors will have seen hundreds of pages with the text and links in their default colors and will take advantage of this conditioning by being able to navigate a new site without having to constantly relearn the interface.
It's obvious that if you change the color of the links on your site, or use an imagemap, much of the possible information will be lost to the visitor and your site will be more difficult to navigate.
Here's another example. If you have a form on one of your pages, there will probably be certain items which are required, and some which are optional. Try putting a red asterisk next to each item that the user must set. This will be a great help in filling out the form since you can instantly tell if it's been completed to the point where it won't produce an error message.
Remember, also, that some of your visitors are color blind. At its worst, colors are varying shades of gray. Colored text against a colored background could show up as a gray smudge. Take this into account when you design your pages.
Other visitors, with less-than-perfect vision, may have trouble viewing images and text with strongly contrasting colors (like red text on a blue background). Don't make your page so that it can only be viewed by teenagers.