Example

function paintOpaqueImage($target_color, $replacement_color, $fuzz, $channel)
{
    $imagick = new \Imagick(realpath("images/BlueScreen.jpg"));

    //Need to be in a format that supports transparency
    $imagick->setImageFormat('png');

    @$imagick->paintOpaqueImage(
        $target_color,
        $replacement_color,
        $fuzz * \Imagick::getQuantum(),
        $channel
    );


    //Not required, but helps tidy up left over pixels
    $imagick->despeckleimage();

    header("Content-Type: image/png");
    echo $imagick->getImageBlob();
}