The difference with Return of the Jedi and The Emperor’s first use of lightning is that he is a new character. It’s established as he’s being established. Red Lightning would be introducing something different to what had been established before. That’s the point. If it had always been red, or always fluctuated colour between characters, this would be different.

And I get what you mean, but there are still different ways of establishing without “spoon-feeding”. Even using the example above, the change is what needs to be established again— because it’s changing something we’re already familiar with. If the Red Lightning is more dangerous, that should impact the stakes of the film, and be reacted to as being more dangerous, but that doesn’t happen. Also, part of the film’s plot revolves around Rey worrying that she is inherently evil. It matters that when she sees lighting directed at her, that it matches the Lightning she used.

Establishing unexplained things as the way they are is simply different to changing established concepts and powers.