Here are some examples which may help you get started, you can pick one and modify it for your site. This example will change the background color for the entire left sidebar in the Mimbo Pro theme:
#sidebar {
background: #ffc;
}
This example will change the background color for all custom menus in the left sidebar in the Mimbo Pro theme:
#sidebar .widget_nav_menu {
background: #ffc;
}
This example will change the background color for just the first custom menu in your site using the ID assigned to the first menu. You can find the ID for any menu by adding it and then looking at the generated page source to see what the "id" value is set to.
#nav_menu-5 {
background: #ffc;
}
Adjust the color as necessary.