Browning happens when you over clean a carpet and change it’s chemical composition.
Detergent based carpet cleaning chemicals are alkaline – some heavily alkaline. Carpets are a little on the acid side of neutral. And, carpet dyes are a little on the acid side of neutral – otherwise they wouldn’t work with the carpet fibers. If you leave the alkaline carpet cleaner in the carpet ( i.e. you don’t rinse enough ) the dyes become inactive and what’s left is usually something between a beige and a coffee colored carpet.
Best (worst case) example I ever saw was a large manufacturing company in the western suburbs that prided itself on it’s ( large and expensive) in house cleaning crew. Every foot of carpeted corridor in the place had 6 inches or so of original color (some version of blue I think) along the walls and a uniform brown thru the middle of the aisle.
A soil pattern, by the way, will NOT be uniform – it will be darkest in the middle and progressively cleaner as you progress out to the edges (thank you professor Bernoulli ? ).
Told the facilities guy the answer –
1) Neutralize the carpet with a mild acid product. You can buy such “neutralizers” at your janitorial supplies distributor to run thru your carpet cleaning equipment. The old fashioned option would be to mist it with a mild vinegar solution. This should immediately acidify the dyes and fibers and restore the original carpet color.
2) Henceforth clean with just water – no chemicals – a few times. There was enough soap residue in that carpet to stop buying chemical for years.
He did. It worked. He got rid of the in – house crew and hired a cleaning contractor – not me unfortunately. C’est le guerre ?