As far as I know "White Scale" is a term with no particular meaning, except the obvious one.

I'm *not* a plaster expert, but I know there have been problems with colored plaster almost as long as there has been colored plaster. And, if there's ever been any agreement on what the causes were, I haven't heard about it. All I've seen are these circular accusations: "It's the colorant! No, it's the cement! No, it's the plasterer! No, it's the homeowner!", and so on.

Blaming the homeowner - which might be valid, but I don't know - is popular, since they aren't in the trade.

Bottom line: do what he says, no matter how much acid it takes, so you'll be able to blame him.

Oh yea, CYA will not help stabilize pH.

Good luck!