colorizing greyscale images whilst preserving alpha

gistfile1.aw

<?php

function colorize($img,$color) {

	$fill = new Imagick;
	$fill->newImage(
		$img->getImageWidth(),
		$img->getImageHeight(),
		$color
	);

	$img->compositeImage(
		$fill,
		Imagick::COMPOSITE_MULTIPLY,
		0,0,
		(Imagick::CHANNEL_RED|Imagick::CHANNEL_GREEN|Imagick::CHANNEL_BLUE)
	);

	$fill->destroy();
}

$img = new Imagick('chip.png');
colorize($img,'#00ff00');
$img->writeImage('chip2.png');
$img->destroy();

?>