Up until the finale of AOTC, green sabres for masters and white sabres for knights/apprentices makes sense. In fact, at the time I was certain Lucas WAS doing that…but then Obi-Wan never used green. Of course, part of that was because white vs white made for a more interesting ROTS.
So it may have just been a happy coincidence up until the end of AOTC.
I think part of it was just that he was boxed in by ANH and had to have obiwan’s saber match. I personally like the battle in ROTS between Anakin and Obi as blue and green sabers and have never been a fan of both having blue. Plus it makes it hard to tell whos swing is whos in some scenes.