Dek Rollins said:
Just because there weren’t any others doesn’t change the fact that it was used to portray evil and to contrast with the good, portrayed by blue. If there was another evil lightsaber-wielder in the OT, he would have had a red blade to keep the feel. A real prequelism is giving all darkside force users the name “Darth” as a title, rather than it being Vader’s first name.
The reason Darth had a red saber in Star Wars was to create the simple contrast of good and evil, just like everything else in the film.
That’s an out of universe reason. There’s no reason to think it’s an in-universe reason going off the OT alone.
Why does it matter if there’s a specific in-universe explanation for it? It’s a specific aesthetic that was used to portray something in the film(s). I never said there was an in-universe reason, I just don’t think something such as this needs one.
When I was a kid I thought his name was “Dark Vader,” which I think supports the idea that the aesthetics were very important to highlighting the contrast between good and evil, if for no other reason than the refined black and red aesthetic of Vader contrasted sharply with the more rugged, “monkish” earth tones and blue saber of Ben—an aesthetic which is repeated in TFA with the Rey-Kylo face off.