Hi Berhard!
The example image is in gif format.
Of course - what do you expect?
That means it only supports 256 colors.
Well, if you try the XDRIVER you get 24bit of course.
Still an PNG picture (not using indexed colors) would be indeed show
24bit. (Sideplug: more people should considere this nice graphic format
http://www.cdrom.com/pub/png/ )
I think I have to spent my time on other problems
more important that creating such examples. If you
don't mind I keep it like that and try further
bugfixing on GRASS5. You are kindly invited to
help - there are still enough problems. 
Best regards
Markus Neteler