I also do not believe it was accidental. Especially for black. It's actually rare to find a NLR black with no AV than with AV. As with the other colors though, blue, bronze, green, yellow red may have been more accidental to experimental. In some cases it's just a matter of the AV spilling into the next color. In some, like a blue zebra you will see the av in the smaller ones but not in the big shooters. Why that is, I do not know. Companies did keep secrets.Their secret may have been in the process of doing consistently and precision.
Here are 2 pics. The first is a real nice precision blue galaxy where the av doesn't stray. The second is what is more common that you see for a blue galaxy and that is what I call a "sloppy" galaxy. The av spreads out and in some parts spills over into the next color. What Ann is showing is a bronze zebra. Was that color accidental or experimental? I will go with the latter.
precision av
sloppy av spilling