Thanks SS113 -- commencing with the magnifying lamp... I'll be back if I don't find any solder splashes or damaged traces (on the top layer of the PCB at least).
-- a quick comment though, wierdly enough, it seems that if you jiggle the cable, or only partially insert it (and hold it there with your hand), you can re-introduce the color. It would seem that the xbox is still sending color data to the cable socket, which makes me think there may be something amiss inside the socket itself, or with the socket's connection to the MB. Just some thoughts, let me know what you think.