If you're looking for a solution to fill the image with a solid color, preserving background transparency, here is one way:

<?php
$im = new Imagick('image.png');
$im->setImageAlphaChannel(Imagick::ALPHACHANNEL_EXTRACT);
$im->setImageBackgroundColor('color');
$im->setImageAlphaChannel(Imagick::ALPHACHANNEL_SHAPE);
$im->writeImage('output.png');
$im->destroy();
?>

Do you want a color overlay with TRUE opacity control? Try this:

<?php

class YourImagick extends Imagick
{
    public function colorize($color, $alpha = 1)
    {
        $draw = new ImagickDraw();

        $draw->setFillColor($color);

        if (is_float($alpha)) {
            $draw->setFillAlpha($alpha);
        }

        $geometry = $this->getImageGeometry();
        $width = $geometry['width'];
        $height = $geometry['height'];

        $draw->rectangle(0, 0, $width, $height);

        $this->drawImage($draw);
    }
}

?>

How to use:

<?php

$imagick = new YourImagick('example.png');

$imagick->colorize('#ffcc00', 0.35);

header('Content-type: image/png');

echo $source;

?>

To improve upon "php at lfbittencourt dot com"'s solution, this one blends the composited color, and takes opacity into account as well.

<?php
class YourImagick extends Imagick
{
    public function colorize($color, $alpha = 1, $composite_flag = Imagick::COMPOSITE_COLORIZE)
    {
        $draw = new ImagickDraw();

        $draw->setFillColor($color);

        $geometry = $this->getImageGeometry();
        $width = $geometry['width'];
        $height = $geometry['height'];

        $draw->rectangle(0, 0, $width, $height);

        $temporary = new Imagick();
        $temporary->setBackgroundColor(new ImagickPixel('transparent'));
        $temporary->newImage($width, $height, new ImagickPixel('transparent'));
        $temporary->setImageFormat('png32');
        $temporary->drawImage($draw);

        $alphaChannel = $this->clone();
        $alphaChannel->setImageAlphaChannel(Imagick::ALPHACHANNEL_EXTRACT);
        $alphaChannel->negateImage(false, Imagick::CHANNEL_ALL);
        $this->setImageClipMask($alphaChannel);

        $clone = $this->clone();
        $clone->compositeImage($temporary, $composite_flag, 0, 0);
        $clone->setImageOpacity($alpha);

        $this->compositeImage($clone, Imagick::COMPOSITE_DEFAULT, 0, 0);
    }
}
?>

The solution published by "olav at redwall dot ee " was adding a black surround outside each non-transparent shape.

Here is my improved version :

<?php
public function colorize($color, $alpha = 1)
{
    $geometry = $this->getImageGeometry();
    $width = $geometry['width'];
    $height = $geometry['height'];
        
    $draw = new ImagickDraw;
    $draw->setFillColor($color);
    $draw->rectangle(0, 0, $width, $height);

    $temporary = new Imagick;
    $temporary->setBackgroundColor(new ImagickPixel('transparent'));
    $temporary->newImage($width, $height, new ImagickPixel('transparent'));
    $temporary->setImageFormat('png32');
    $temporary->drawImage($draw);
    $temporary->compositeImage($this, Imagick::COMPOSITE_COPYOPACITY, 0, 0);
        
    $this->setImageArtifact('compose:args', ($alpha * 100) . '%,100%');
    $this->compositeImage($temporary, Imagick::COMPOSITE_DISSOLVE, 0, 0);
}
?>

When you're using an image with an alpha channel (for example a transparent png), a value of 1.0 will return a completely transparent image, but a value of 1 works just fine.

simplest example

<?php
$nombre = '001-4-0043.jpg';
$img = new Imagick($nombre);
$img->negateImage(false);
//$pixblu = new ImagickPixel('#000040');
$img->colorizeImage('#0000b0',1.0);
header('content-type: image/jpeg');
echo $img;
?>