test.php

<?php

$imagick = new Imagick();
$imagick->readImage('image.jpeg');

$it = $imagick->getPixelIterator();

$width  = $imagick->getImageWidth();
$height = $imagick->getImageHeight();

$h = 1;
foreach($it as $row => $pixels)
{
    $save = ($h == 1 || $h == $height);

    $w = 1;
    foreach($pixels as $pixel)
    {
        if($width == $w || $w == 1) {

            if ($save) {

                $corners[] = $pixel->getColorAsString();
            }
        }

        $w++;
    }

    $h++;
}

$p = array_shift($corners);

foreach($corners as $c) {

    if ($c != $p) {

        die('fail not the same color');
    }
}

$imagick->paintTransparentImage($corners[0], 0.0, 0.0);

$imagick->setImageFormat('png');
$imagick->writeImage('image.png');