I suspect your thinking along dc rules and that our comm. has something to do with earth ground safety or something; it doesn't. We should really almost be calling "hot&comm" "variable or constant & return(s).
I can apply almost every question and statement made to various scenarios. So either your question isn't clear in context, moves subjects without notice, or your missing something, or more precise, expecting something that isn't there. You haven't spoke much on what you DO know, so we don't know what your missing or what's wrong.
Phasing is for transformers. Don't skip it unless you like 50v-60v on your layout. Nuff said on doing it, MARK IT when done. (new ones wont even turn on till this is done )
Now focus on how to draw what you need. Some conditions may call for odd phasing....But, once pre phased, that just means unplugging a transformer, flipping the plug and plugging it back it. 5 seconds. You just learned how to test. Change your choice of common can be done with more tests.
If you want to learn, focus on the thoery part of the lionel phasing video. Then the mechanical aspect becomes easy. (or stick to exactly what they tell you how to do)
If you mean a bulb used in a phasing test somehow you didn't say so, and the question & your example needs some rephasing
A bulb is nothing more than a burn out resistant wire in a vacuum to make in even harder to burn up.(no oxygen) It doesn't care if the power moves left acoss the filement wire (lets say thats dc+) or right across the wire (leaves dc-); or if it moves Lft then Rgt really fast (thats ac)
That said, wire color may matter for a socket design.
red & black comming off a bulb socket is sometimes oriented to CALL attention to the orientation of the socket's 2 contacts to a mounting screwor bracket. Or to match a system "all pretty" (think of some of the uneeded efforts you see for a "pro look")
Real reasons are also likely to be a mechanical based preference more so than any electrcal reason alone. E.g.--if mystery wire #A rubs raw on surface #B, will there just be bypass, smoke, or a fire?
There is not, nor has there ever been a common, two contact, wire in vacuum incandescent light bulb, that cares about ac polarity; because in ac, polarity changes. (doesn't count on all 3+ terminal bulbs, just some.)
Now, a model light post's platform base is often part of power delivery to a bulb and there are concerns to be talked about if that base isnt on the main com. but bumps a main common (or other tap)