Seems that this method doesn't work as expected. Use Imagick::floodFillPaintImage() instead.
<?php
$img = new Imagick('/tmp/test.png');
$img->floodFillPaintImage('green', 10, 'white', 100, 100, false);
$img->writeImage('/tmp/out.png');
$img->clear();
$img->destroy();
?>