Lilmod:
You have two statement that seem to contradict each other…
1. When I said that it’s untznius, I did not mean that it’s assur. To my knowledge, it’s not assur. I merely meant that if you are asking about it, I would certainly say that it is a higher level of tznius not to wear it.
2. If it’s not assur, why assume that nail polish is untznius?
So why do you say black is “not Tznius” if you agree it isn’t Assur?