Imagick convert SVG to PNG - colors replaced by black and white

Imagick convert SVG to PNG - colors replaced by black and white

I'am trying to convert an SVG Image which is created by the SVGGraph
library ( http://www.goat1000.com/svggraph.php).
The SVG is colored (red, green, yellow, gray, ...) in the browser and
everything is fine. But when I convert it, its just black and white.
With this code I convert it:
//new instance of imagick
$im = new Imagick();
//read the svg file/data (its not saved on the filesystem)
$im->readImageBlob($svgFile);
$im->setImageFormat("png24");
$im->writeImage('cylinder.png');
$im->clear();
$im->destroy();
I've tried it with jpeg and png as output format but the result is the
same, all colors will be replaced with black
does anyone have an idea how to fix that ?