Part of me likes the idea that it’s the midway point between blue and red, but also thematically there isn’t really anything in the movie to set that up. Also the fact that Mace has a purple saber obscures that idea.
I read a theory I really like that suggests color distinguishes alignment -
Red: Extremist dark side user (ex: Maul)
Orange: Dark side user with a significant light side pull
Yellow: True balance (ex: Rey)
Green: Roguish light side user (ex: Qui-Gon)
Blue: Traditional light side user (ex: Obi-Wan)
Purple: Extremist light side user (ex: Windu)
Now that I think about it, I probably found it here. Forget where it’s from lol.