Did you know that some rugs and mats permanently discolour your carpet / flooring that you put them on ?
This is usually caused by a chemical reaction that occurs because of a lack of sunlight. The technical reason is explained as ( butylhydroxyltoluene ), which is commonly used in the backings of rugs & mats, will let off a gas. The gas gets trapped underneath and causes the yellowing we so often see.
Once the discoloration has started, it can not be reversed in my experience.
To help prevent this happening, it is best to move your rug once in a while to let the carpet / floor underneath breath.