Sometimes newer TVs have 2 settings for each AV channel. One is normal and one is S-Vid. On my Sony widescreen, I need to have it on the S-Vid when using the adaptor otherwise the display is in black and white.
Also, it needs to be in scart socket 1 or 2 as it does not work in the 3rd scart socket.
Try pressing AV again or go through the TV settings to see if you can enable S-Video.
Alternatively, some TV's have an S-Video socket in the front behind the flap where the setup buttons are.