We want to keep the fights as readable as we can. This is largely why we've elected to keep the sith lightsaber colours restricted within shades of red.
Seeing one Anakin Skywalker with a purple lightsaber and another Anakin Skywalker with a purple lightsaber and having to guess which team each belongs to at a glance is something we don't want to have.
In a game where team damage is a core concept, it is key to keep situations easy to interpret. We've already failed on this from a certain standpoint with having very similar character designs (clones/stormtroopers, some jedi/sith) on both teams, but that is something too deeply embedded into the game for us to change at this point without stirring up a lot of trouble.
Crosshair identification alone does not suffice in a game where the player relies on fast reaction times and decision-making to succeed. You need distinguishable silhouettes, animations, colours and sounds.
TL;DR: It's not about canon, it's not about us hating on your OC sith character that you like to RP, it's simply about game design 101.
Worth noting that adding black/white sabers for sith and jedi respectively is not out of the question, but currently not a priority. The current implementation of lightsabers uses an RGB system where we have one fully white sprite texture for all lightsaber blades, the color of which we manipulate on the shader level with game code. In the shader absolute black equals transparent, thus it's not possible to have a black with the current implementation.
Implementing black in particular is as a result very dreary and if no programmer feels like doing it, I think it's fair enough to not force them to work on something that niche.
Only way for purple to get in would be for us to do a very strict pass on all jedi/sith character models. This would cull out a lot of the characters so we can make characters very clearly distinguishable. Force users with black outfits -> Sith, every other sith removed. Force users with light brown outfits -> Jedi, every other jedi removed.
So do you want purple lightsabers for sith or do you want to keep playing as Ep3 Anakin on both teams? Would you be okay with no Ep6 Luke Skywalker on jedi?