Turn off auto-grayscale when saving to png with PHP Imagick

When saving a RGB image to a PNG with Imagick. The image gets saved automatically in grayscale when it contains only grey pixels. I understand this a default stetting, that can be overridden. I would like to save my image always in RGB as a default. How do I turn Imagick’s auto-grayscale off with PHP/Imagick? I wrote this code.

$im = new Imagick();
$im->setResolution(288,288);
$im->setColorspace(Imagick::COLORSPACE_SRGB);
$im->setBackgroundColor(new ImagickPixel('transparent'));
$im->readImage($orgDataPath);
$im->setoption("colorspace:auto-grayscale", "off");
$im->setImageType(Imagick::IMGTYPE_TRUECOLOR);
$im->trimImage(0);
$im->setImageFormat("png");
$im->writeImage($pngPrint.".png");
// cleanup
$im->clear();
$im->destroy();