Have you ever played City of Heroes? It was an MMORPG that released in the same year as WoW (I think a bit earlier in the year).
Anyway, City of Heroes was, and remains, God Tier in terms of character customization. Even back then, players could change the colorization of their spell effects and abilities. You had the literal rainbow spectrum to choose from, and some effects could have a secondary color, but otherwise your entire power suit matched the same color scheme (it was your Super Hero signature).
I said that to illustrate that the tech is there, but virtually no MMORPG has dived that deeply into character customization since City of Heroes, and it’s a shame (ESO comes the closest, IMHO). Even if Blizzard didn’t want to give players THAT much control, I sincerely doubt there’s anything preventing them from adding void effects to spells.
They themselves used that control once upon a time, when different races HAD different priest spell effects. However, spell effect coloration may be a greater discussion overall, as the option to do this would open up a huge world of possibility (and headache) that Blizzard may not want to dip into immediately.