If there was a white city it must have had limestone buildings.
It wouldn’t have stayed white for long if it wasn’t, in a damp climate.
Limestone, calcite, whatever was around, they used it. I’ve never seen it written, but I suspect it has been — the exteriors of the pyramids and other large structures were basically stuccoed and frescoed, and over the centuries of abandonment all of that fell away in the rainy seasons.