Example

function resetIterator($image_path)
{
    $imagick = new \Imagick(realpath($image_path));

    $imageIterator = $imagick->getPixelIterator();

    /* Loop through pixel rows */
    foreach ($imageIterator as $pixels) {
        /* Loop through the pixels in the row (columns) */
        foreach ($pixels as $column => $pixel) {
            /** @var $pixel \ImagickPixel */
            if ($column % 2) {
                /* Make every second pixel 25% red*/
                $pixel->setColorValue(\Imagick::COLOR_RED, 64);
            }
        }
        /* Sync the iterator, this is important to do on each iteration */
        $imageIterator->syncIterator();
    }

    $imageIterator->resetiterator();

    /* Loop through pixel rows */
    foreach ($imageIterator as $pixels) {
        /* Loop through the pixels in the row (columns) */
        foreach ($pixels as $column => $pixel) {
            /** @var $pixel \ImagickPixel */
            if ($column % 3) {
                $pixel->setColorValue(\Imagick::COLOR_BLUE, 64); /* Make every second pixel a little blue*/
                //$pixel->setColor("rgba(0, 0, 128, 0)"); /* Paint every second pixel black*/
            }
        }
        $imageIterator->syncIterator(); /* Sync the iterator, this is important to do on each iteration */
    }

    $imageIterator->clear();

    header("Content-Type: image/jpeg");
    echo $imagick;
}