I think the total light output of each bulb in the pair is the same at all points in time, but the orange-blue gradient is reversed. So when one is orange at one end, the bulb beside it is blue at that end.

IIRC, the end that's negative looks orange, because the electrons emitted from the filament haven't gotten up to speed yet and can't ionize the mercury atoms at that end to the highest states.

If you didn't do this, you'd see 60 Hz strobing when you looked at one end.