You'll probably also want to change the hover color for menu items. Here is a set of CSS you can start with that contains all of the color-related rules for the main menu in the Twenty Ten theme:
#access {
background: #999;
}
#access a {
color: #000;
}
#access ul ul {
box-shadow: 0px 3px 3px rgba(0,0,0,0.5);
-moz-box-shadow: 0px 3px 3px rgba(0,0,0,0.5);
-webkit-box-shadow: 0px 3px 3px rgba(0,0,0,0.5);
}
#access ul ul a {
background: #999;
}
#access li:hover > a,
#access ul ul :hover > a {
background: #333;
color: #fff;
}
#access ul li.current_page_item > a,
#access ul li.current_page_ancestor > a,
#access ul li.current-menu-ancestor > a,
#access ul li.current-menu-item > a,
#access ul li.current-menu-parent > a {
color: #fff;
}
* html #access ul li.current_page_item a,
* html #access ul li.current_page_ancestor a,
* html #access ul li.current-menu-ancestor a,
* html #access ul li.current-menu-item a,
* html #access ul li.current-menu-parent a,
* html #access ul li a:hover {
color: #fff;
}
Replace the color codes with colors of your choice.