Hi again, sorry for taking so long to get back on this - I haven't
been around my MX so I could test this myself.
The manual doesn't elaborate too much on this, but the way it should
work is that you assign a "S-VHS" capable source to either of the AV1
and AV2 inputs (you can use at least V.TAPE-1 or V.TAPE-2). Then you set
the "S-VHS" field to the respective socket, AV1 or AV2.
For testing, I now have my MX6000 setup as AV1 = V.TAPE-1 and S-VHS =
AV1, a S-video source connected to the S-VHS input, and I get a good
coloured picture when I select V.TAPE on the Beo4 (testing AV2 would
require quite a bit of juggling, so I didn't do that...).
If that still doesn't work for you, you might have a setup problem on
the iMac output (have you ever seen it in colour in some other setup?).
Unfortunately I'm not able to help you with that at all...
BTW, have you stored the socket setup with pressing "GO" instead of
just exiting the menu?
EDIT: the S-video socket in my MX is extremely tight. Verify that you have the plug properly inserted.