I found a thread from September where it was said that this might be supported in future, and I assume that didn’t happen in 3.1. Is there a workaround? I can’t seem to change the colour to white, or end/begin the voice. Thanks.
I think I answered my own question. Notation Options -> Inactive Players -> Hide rests and label active player
Though it seems to be per-Flow (which is fine for my current needs).