I think yer S-Video cable is dodgy and only transmits the brightness infromation instead of both brightness (luminance) and chrominance.
You can force a composite signal over S-Video.
To do this bring up the nVidia applet and go to device settings for your TV as shown here
and then change the TV output type to composite instead instead of S-Video or Auto as shown here:
Hopefully that should give you a colour signal over S-Video.