There is a phenomena that occurs beneath area rugs that lay on installed carpet. At least three factors contribute. 1) The fibers of installed carpets are typically glued to the backing. The glues offlays and this causes a shift in the PH balance of the carpet, which over the course of 4 or 5 months often develops a yellow tinge in the carpet beneath the area rug. When this happens, it is usually irreversible. 2) The light usually comes in the windows and dulls the carpet surrounding the area rug. When the area rug is pulled back, you then have a nice bright rectangle where the area rug was (95% of the time). 3) Dyes from area rugs are often unstable and bleed into the carpet underneath, especially along the edge of the area rug.