Add a Tag into WordPress' RSS Feed

<?php
    /**
     * Plugin Name: Extend RSS2 Feed
     * Author: Websupporter
    **/

    add_action( 'rss2_item', 'erf_rss2_item' );
    function erf_rss2_item(){
        $farbe = get_post_meta( get_the_ID(), 'farbe', true );
        if( ! $farbe )
            return;
        ?>
        <farbe:wert><?php echo $farbe; ?></farbe:wert>
        <?php
    }

    add_action( 'rss2_ns', 'erf_rss2_ns' );
    function erf_rss2_ns(){
        ?>
        xmlns:farbe="http://websupporter.net/ns/farbe/"
        <?php
    }
?>