Are you sure that's the original colour?
I've cleaned some carpets where they have been made up of strands of several different colours. Over time, the strands of a certain colour have degraded and broken away so the remaining colour stands out.
Very difficult to tell from your pic exactly what's going on, but an interesting one all the same... can't say I've ever encountered such a colour change simply from the carpet just being "cleaner" at the end of the job!