This will change EVERYONE'S name color to default...
<br />UPDATE w3t_Users<br />SET U_Color = ''<br />
Starting with 6.5, admin and mod colors will be handled by stylesheets. I don't know if this is the case for normal users as well.
If you're using < 6.5, to reset titles, go to your admin section and click on "Edit titles" under the "Filters" category. Check the box and click update to reset everyone.