gray scale image with Magick++ API

Hi,

I tried to convert color images to gray images with the following code:

Magick::Image image;
image.read("color.jpg");
image.colorSpace(GRAYColorspace);
image.write("gray.jpg");

The resulted image seems to be exactly as the original image. What's
wrong with my code?

Thank you for your help in advance.

I also tried this without success:
Magick::Image image;
image.read("color.jpg");
image.monochrome(true);
image.write("gray.jpg");

Best regards!