Questions about the specifics of choir dress of ICKSP canons, Part II: do they all wear fully black cassocks in choir, or with blue buttons & piping?

My previous question pertained to the weird manteletta/mantellone hybrid the ICKSP canons seem to wear in choir, but here I would like to focus just on the cassocks. There is some conflicting information circulating regarding the type of cassock the ICKSP canons wear in choir - some sources show a plain, fully black cassock, while others suggest a black one with aqua blue buttons and piping, like the one Prior Genral Wach is wearing here as his formal outfit (with a matching pellegrina, fascia and even a ferraiolo, no less): The priests accompanying him have fully black cassocks, fascias and ferraiolos, with no pellegrina, and I'm assuming they're also of the ICKSP because of the ferraiolo (a sign of travel or envoy). And that appears consistent with what I've seen of the canons in choir dress too - fully black cassocks. I've perused many photos showing both canons and superiors, and I was unable to find a single one where blue buttons/piping would be visible (where it was even possible to see the front of the cassock, which was usually hidden by the mantelletta), in fact they seemed to be fully black at all times. One exception would be one of the Superiors, who wore a cassock with purple buttons and piping ("traditional" canon/Chaplain of his Holiness cassock) in one picture - somewhat coloristically jarring, but understandable. So is it that only the Prior General is entitled to a blue-laced black cassock as a sign of his authority, while all other canons (including superiors) wear fully black cassocks on all occasions? That would make him mirror a bishop in the difference between a piped normal cassock and a fully-colored choir one.