Nav Menu – Add class based on meta keys

Your code doesn’t because the global $post refers to the post/page being viewed (if there is one…). What is passed with nav_menu_css_class is a menu item – this is a particular post type. Among its properties are:
object which stores:

• the post type if the link refers to a post
• the taxonomy if the link refers to a taxonomy-term page
• ‘custom’ if its a custom link

It also stores the object_id

• the post id if the link refers to a post
• the term id if the link refers to a taxonomy-term page
• the post id of the custom link

So in what you want to do is check if the term actually refers to a post (or a custom post type), and then pass the object_id to the get_post_meta():

function wpse51076_special_nav_class($classes, $item){
    if( 'post' == $item->object ){
            $colorcode = get_post_meta($item->object_id, 'color_dropdown', true);
           $classes[] = $colorcode;
    }
    return $classes;
}
add_filter('nav_menu_css_class' , 'wpse51076_special_nav_class' , 10 , 2);