Yes, I had this very same problem when I bought my Xbox a few years back.
At the time I had a Sony Trinitron 20" TV and was horrified when I saw Halo in black and white. You will most likely need to buy a Digital Scart cable. The cable supplied with the Xbox (and PS2) is an analogue cable with a Scart adaptor on the end.
I phoned Microsoft at the time and they told me that my TV type wouldn�t work correctly with an analogue Scart cable. Sounded a bit dodgy to me as I had a TV from their main competitor!!
This is most likely your problem.