My feeling is that color code changes should only be required for new construction, or major remodels, but that limited additions to an existing installed system should reflect the code currently in use within that system, especially if old and new codes use the same colors for different purposes.
If the wiring is in cable, it's one thing, since you can tell that the wiring is matched to the new or old code, but in conduit, it's a different story.
It should never be possible to have 2 wires of the same color, in the same pipe, with opposing functions, as that is a recipe for disaster.
