try to use a tool such as Firebug http://getfirebug.com/ to find which styles effect the menu item color.
when the theme uses the custom menu option, this is usually .current-menu-item
how specific your new style has to be, depends on the existing ones;
try for example:
#navi ul li.current-menu-item a { color: green; }
Thanks, however, I can get it to change color on the hover, I want it to STAY that color when I click on it.