When dealing with accessibility issues, one request that I often run across is setting colors for varying degrees of color blindness. Many people have some form or another of color blindness that prevents them from seeing color as the rest of us.  Some vision deficiencies make it very difficult to distinguish web colors.

Changing monitor brightness and contrast help, but as a designer I also look for compatible or non-compatible colors to aid these conditions. Complicating the issue a little more…colors render differently between Windows computers/montors and Apple/Mac computers/monitors.