One of the most common accessibility barriers I come across in my daily browsing is that of colors being only partially specified.
The W3C is doing a good job of spreading the word by including warnings in the CSS validator, and Tips of the Day in the HTML validator, but they seem to be ignored in many cases.
These warnings should be heeded, not necessarily to adjust your code so there are no warnings, but to realize there is a potential for conflict.
If you use the default color schemes for your operating system and browser, conflicts may not be apparent in a visual check.