Before tackling the question of one-way arrows, I think we should first decide on the overall goal we want to achieve with our decision. For me, the main question is:
In the case of interactive objects like OW-arrows, teleporters, radiation/slowfreeze objects, splitters, buttons, ... do we want
1) a uniform design over all level, or
2) lots of differently looking objects that have the same function?
Choice 1) would make it easier for new players to recognize what the objects do. It would simply remove any questions like: "Is that greenish-blue ball in the
horror wasteland tile-set a continuous trap, a radiation object, a teleporter or an (anti)-splat pad?" Instead there would be a set of clearly distinguishable objects (not like the sparkling that sometimes is slowfreeze and sometimes radiation).
Choice 2) would give much more artistic flexibility to graphic style designers, because they can adapt existing objects so that they fit the respective theme.
Once we answered this question, making a choice for one-way arrows is much easier:
If we want to go into the direction of choice 1), then having a uniform style as in option A is the way to go. But we certainly have to allow for lots of different colors, so that the arrows stay clearly visible for the various styles with their different main color schemes.
If we decide to head into the direction of choice 2), then option B fits very well, because it allows the same flexibility than we would get for any other object.
I myself would push for a uniform style for interactive objects (except for exits, traps and possibly hatches), just because I had problems recognizing some of the objects in the past. And if today's comment in IRC
[15:25:29] <namida42> Flopsy: re level 14, that weird object is a receiver
is anything to go by, then I am not the only one with this problem.
And I certainly wouldn't miss some of the custom one-way arrows, like the one for the psychedelic style (which is not very visible on terrain), the clockwork one (which isn't really different from the standard arrows, except that they sometimes move in the wrong direction) or the (custom) menace one (which is a very deformed and not really recognizable arrow).