Additional addendum, it seems a non indexed PNG with transparency works, which is unique behavior as most everything else seems to require the index to match up. I haven't tested with non standard colors, but it seems to be failing gracefully based on my preliminary test.
EDIT. Let me know if I'm wrong please, but now that I think about it, there's not a pallet for the base specifically, unless it's using battlescape? maybe it's looser than all the other sprite requirements? I'd just gotten in the habit of using the battle pallet set for most of this stuff since I've not touched UFO pedia or other items much yet.