Only Mantra is Haki
Only Rokugan is Haki
They are both Haki
They are both NOT haki
I'm not against haki being visual, I'm against haki becoming visual for no reason. Make it clearer for the reader is not a reason to create inconsistency in the manga. It's not like Oda is a newbie and has never read how invisible forces are treated in manga history. He choose to make haki invisible in the beginning, he had probably his reasons so he should stay to his initial decision.
Plus if we agree that characters from a certain level of strength knows haki, then there is no need to display it by a specific colour.
It's only a problem for people that accept something only when it has been explicitly written by Oda.