Index: custom-background.php
===================================================================
--- custom-background.php	(revision 28718)
+++ custom-background.php	(working copy)
@@ -204,9 +204,9 @@
 			set_theme_mod('background_attachment', $attachment);
 		}
 
-		if ( isset($_POST['background-color']) ) {
-			check_admin_referer('custom-background');
-			$color = preg_replace('/[^0-9a-fA-F]/', '', $_POST['background-color']);
+		if ( get_theme_support( 'custom-background', 'default-color' ) == $color ) {
+			remove_theme_mod( 'background_color' );
+		} else {
 			if ( strlen($color) == 6 || strlen($color) == 3 )
 				set_theme_mod('background_color', $color);
 			else