You might need to set the colorspace the same when composing two images over each other
<?php
//Creating two Imagick object
$first = new Imagick('first.jpg');
$second = new Imagick('second.jpg');
// Set the colorspace to the same value
$first->setImageColorspace($second->getImageColorspace() );
//Second image is put on top of the first
$first->compositeImage($second, $second->getImageCompose(), 5, 5);
//new image is saved as final.jpg
$first->writeImage('final.jpg');
?>