Ya know, aesthetically, I like this. It’s black and red; what’s not to like? But in terms of building a characterization consistent with an entire civilization I can understand why you don’t use it.
Threatening colors in nature is called aposematism. Wiki calls it, “advertising” to predators that the animal is not safe to eat. Basically the idea is that the colors make you look dangerous. It’s something we, as animals, have internalized. That’s why we use those colors
the way we do in designs. It’s instinctual that we understand the threat aspect, and we use those colors ourselves when we want to intimidate others. By doing that we are basically labeling ourselves as prey. We’re saying, “you better stay away from me cause I’m BAD NEWS”.
We understand that the colors are threatening but we misinterpret why so we use them backwards, essentially coloring predators to intimidate prey - which is not something you really want to do.
Steppenwolf is a predator, so by that nature, as trying to create an internal logic for the character, he wouldn’t naturally develop aposematism and wouldn’t feel the need to replicate it in his look, if he could even choose to.
I’m not saying it’s the, “correct” approach. Black and red are cool to look at and they get attention. But from a naturalistic perspective, if you’re doing more than designing a character just to, “look cool”, I can see why you might not go that route.