Effective Use of Colors
The rules for effective use of color in style sheets are the same as those for web pages in general: for a consistent appearance, you should use the 216-color safety palette, as I explain in the HTML Style Guide. You should avoid changing colors for every selector simply because it's easy to do it. Color is a powerful visual signal to mark different kinds of information, but only if it used carefully and consistently.
Beyond that, you should be careful about the use of background and foreground colors if you think that users may want to print your pages out from a browser. Light colored text on a dark background often looks good on a display, but will probably print out as a solid black rectangle on a black and white printer.