On my TV, colour is very hard to detect using s-video from the Pandora - window titles are very slightly blue, and only the volume indicator is identifiably blue. On composite I get better colour.
It may be that the luminance connection to the s-video port is acually luminance+chroma i.e. composite, I don't know how the TV-out cable is wired. Looking at the EXT-connector pinout, the port actually only outputs s-video (i.e. separate Y and C), but depending on how the cable is wired, it may be corrupting the discrete luminance output (Y) on the s-video line, dunno. I'd need an oscilloscope to check, and I don't have one.
Edit: Oops, I hadn't actually gone to the configuration screen and chosen s-video. Now I've done that, colour is much better.
It wouldn't surprise me if s-video over French Peritel is different to SCART, as it was never completely officially part of the standard, as far as I know. However, plugging your s-video plug directly into a TV should work in colour, as s-video is just s-video - Y, C and two grounds. Not much you can do to that and still get a picture.