What was technically known as the Aft Skirt Thermal Curtain (ASTC) was a structure of nine layers of woven insulated material that was a little over 2.5 inches thick. The ring around the RSRM nozzle was made up of 24 segments hand-sewn together.
As best I recall, the colors changed during the course of the shuttle's history; I was able to find photos of the Atlantis mission I modeled and the ASTC on its RSRMs was a tan color.
It is dense reading, but here is one technical paper on them:
https://ntrs.nasa.gov/archive/nasa/c...9940012651.pdf
And, oddly enough, unused segments show up on eBay from time to time:
https://www.ebay.com/itm/Space-Shutt...p2047675.l2557