Fiber damage. I don't like to do inspections, but occassionally I do one for political reasons (as in "industry politics", not national politics!).
The last one that required the microscope was a carpet in the University of Pittsburgh that had a pronounced green discoloration throughout the traffic areas. To the naked eye, it looked like a cleaning related issue.
However, the pattern of the carpet used three colors: Mauve, gold, and green. Under the pocket microscope you could easily see the mauve and gold yarns were badly deteriorated, and only the green was completely intact.
Thus the "green discoloration".