Index: wp-admin/custom-background.php
===================================================================
--- wp-admin/custom-background.php	(revision 22001)
+++ wp-admin/custom-background.php	(working copy)
@@ -169,7 +169,9 @@
 		if ( isset($_POST['background-color']) ) {
 			check_admin_referer('custom-background');
 			$color = preg_replace('/[^0-9a-fA-F]/', '', $_POST['background-color']);
-			if ( strlen($color) == 6 || strlen($color) == 3 )
+			if ( get_theme_support( 'custom-background', 'default-color' ) == $color )
+				remove_theme_mod( 'background_color' );
+			elseif ( strlen($color) == 6 || strlen($color) == 3 )
 				set_theme_mod('background_color', $color);
 			else
 				set_theme_mod('background_color', '');