Alan,
It is no more a grounding conductor than is the grounded conductor in a single phase 120/240 volt system. As far as a grounding function, again it is exactly the same as the single phase 120/240 volt system. The grounded conductor is used for both functions on the line side of the service disconnect. As to your question about the color of this conductor, it must be identified per Article 200.
The circuit to the equipment served by the corner grounded system needs to have two ungrounded conductors, one grounded conductor (identified per Article 200) and an equipment grounding conductor.
That being said, it is rare to find a corner grounded system with the grounded conductor correctly identified.
