PHP function imagick generates grayscale difference pictures

$image = new Imagick('cyclops.png');
$clone = $image->clone();
$clone = $image->fximage('p{0,0}');
$image->compositeImage($clone,imagick::COMPOSITE_DIFFERENCE,0,0);
$image->modulateImage( 100, 0, 0);
header( 'Content-Type: image/{$image->getImageFormat()}' );
echo $image->getImageBlob( );
writeImage('difference.png');
$image->clear();
$image->destroy();
?>

Source image:

Rendering (generated grayscale difference picture):