I have had more than a few S-Video cables lose the chroma signal (since they are separate lines in an S-Video cable) and result in a b&w picture. Those tiny pins are not as robust as one would hope. I remember getting a really nice VHS recorder from eBay years ago that had a bad S-Video connector. In order to get it to work properly, I had to get a rubber band and put lateral pressure on the plug so the chroma signal would make a conection.
When I read this thread title, I thought you might be trying to use the AV1 connector as an input using a SCART adapter. It is not completely wired properly as an input, and will only allow a poor b&w image as an input. It is really supposed to be just an output. As an output, it works perfectly (again, with a SCART adapter). AV2, with the adapter, works perfectly as an input.