Wrong. Smoke or flames have never been associated with ACoC.
The manga told us the first time Zoro used enma what the smoke mean. It just means he's flowing an overwhelming amount of CoA haki to his sword(s).
It has nothing to do with the strength or hardness of his CoA haki.
The different color of the flames/smoke tells us whether Zoro is one in 100% control or not. Green flames/smoke means Zoro is it 100% control. Purple flames/smoke means enma is forced some of his CoA haki out.