The fuzz is just working well in a range of 0 to 65535.
I suggest you to try to move fuzz on a color spectrum image.
1/ Get a color spectrum ( Google Image has a lot )
2/ Try this code :
<?php
    function fuzzTest($source, $target, $fuzz) {
        $im = new Imagick($source);
        $im->resizeImage(320, 240, Imagick::FILTER_LANCZOS, 1, true);
        
        $im->paintTransparentImage($im->getImagePixelColor(0, 0), 0, $fuzz);
        
         $im->setImageFormat('png');
        $im->writeImage($target);
        $im->destroy();
         
        return true;
     }
     for ($i = 0; ($i <= 10); $i++) {
         fuzzTest('spectrum.png', "test_{$i}.png", (6553.5 * $i));
         echo '<img src="test_' . $i . '.png" /> ';
     }
 
 ?>