Ok, pics...........
This is in Hythloth, level 2.
These were added a few years ago, but I never saw what they were for.
This is again in Hythloth, on level 4. Note the color, if I remember right they used to be the standard color like on level 2. They were added at the same time.
This is in the Tomb of Kings. These seem to be bleeding through from another level or something.
And these are all I have seen, as far as I can remember. But there light be more, just don't remember right now.
Also, curious that this possible scenario might mean something:
Hythloth is on Fire Isle, where the entrance to the Underworld arose. The color of the set of Fittings on level 4 of Hyth might have changed when the underworld entrance rose, but not sure about that.
The Tomb of Kings is the Underworld entrance from the Garg lands side.