If you are referring to the lighter band across the width of the carpet, that is abrash. It was there all along, often intentionally sometimes accidentally. Woven with slightly different shade of yarn.
It is considered a sign of better value and "proof" that a rug was handmade.
It may have been hidden by soil or simply not noticed until examining the rug after cleaning. Checking for abrash and photo documentation before cleaning is important,e specially for people who don't know rugs and think the cleaner did something to cause this.
Well, I've learned a new word. I read a few articles on it just in case I ever run into it. I understand the thinking behind the deliberate defect, but I don't think I'd like it as an owner or as a cleaner who happens to "reveal" the abrash. I found several excellent articles on it, but they mostly state the same facts. This one was pretty thorough. https://www.abc-oriental-rug.com/abrash-coloration-in-rugs.html