Paint chips and pigment fragments can last for many years. The components of paint can be identified by archaeologists centuries later. It is possible that there were identifiable paint fragments or chips buried with human remains. It is reasonable to assume that investigators want to check if there's a match between paint evidence found on victims to paint at Heuermann's house.
I seem to recall that the investigation of the Green River Killer (GRK) developed information from microscopic paint fragments found with the victims, and that the paint was a vehicle paint, probably a truck paint. The GRK, Gary Ridgway, worked in a Kenworth truck assembly plant.
As far as the rope is concerned, people may have spools of rope stored for years, depending on how much rope they use. If a burial site yielded some rope, the investigators may want to check if any rope at Heuermann's house is a match for what was found.