Check before publishing, if already exist post with current custom field value

Question

I have a custom post type with only 3 custom fields in it.

$post_types = get_post_meta($post->ID,'post_types',true);
$post_taxonomies = get_post_meta($post->ID,'post_taxonomies',true);
$post_terms = get_post_meta($post->ID,'post_terms',true);

I want to check if already exist post with this $post_types value, and if exist -> show message without publishing.

I created a metabox for this custom fields, which already have function for save_post action hook.

function sidebars_meta_save( $id )  {  
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;  

    if( !isset( $_POST['sidebars_nonce'] ) || !wp_verify_nonce( $_POST['sidebars_nonce'], 'save_sidebars_meta' ) ) return;  
    if( !current_user_can( 'edit_post' ) ) return;  

    $allowed = array(  
        'p' => array()  
    );      

    if(isset( $_POST['post_terms'] )){
                    //check_exist_term - simple boolean function which check if 
                    //a post with this term already exist
        if(!check_exist_term($_POST['post_terms'])){
            if( isset( $_POST['post_types'] ) ){  
                update_post_meta( $id, 'post_types', wp_kses( $_POST['post_types'], $allowed ) ); 
            }
            if( isset( $_POST['post_taxonomies'] ) ) { 
                update_post_meta( $id, 'post_taxonomies', wp_kses( $_POST['post_taxonomies'], $allowed ) );
            }
                update_post_meta( $id, 'post_terms', wp_kses( $_POST['post_terms'], $allowed ) );
            }
        }else{
                        //if I will use wp_delete_post here, the message will not show
            //wp_delete_post($id);
            //wp_redirect('post-new.php?post_type=sidebars');
            //exit;
        }
    }
}  
add_action( 'save_post', 'sidebars_meta_save' );  

My redirect location function :

function sidebars_redirect_location($location,$post_id){
if( isset( $_POST['post_types'] ) ){  
    if(check_exist_term($_POST['post_terms'])){
        $status = get_post_status( $post_id );
        $location = add_query_arg('message', 21, $location);
                    //if I will use wp_delete_post here, I will be not able 
                    //to return to post edit page, because page will be already deleted
    }
}
return $location;
}
add_filter('redirect_post_location','sidebars_redirect_location',10,2);

My post updated messages function :

function sidebars_custom_messages($messages){
$messages['sidebars'][21] = 'Looks like you tried publishing a post with term which already exist!';
return $messages;
}    
 add_filter('post_updated_messages', 'sidebars_custom_messages');    

1) Where I should use wp_delete_post() If I want to first of all to show a message that this post already exist and just then delete this post and stay on this page? 2) How to show red message instead of yellow?