Shading / 'Pile reversal'
At times, certain areas in a carpet may exhibit some color distortion: they may appear darker than the carpet from one angle and lighter from another. This may occur when some pile fibers in the carpet donot lay along the general direction of the pile, causing the light to be reflected at a different angle.This shading or 'pile reversal' is commonly observed in high-traffic area, after use or cleaning.
This is a gradual process, but may become more noticeable just after cleaning, causing some people to presume that it was caused by cleaning.
This process is usually not treatable, though it can be slowed down by ensuring that the pile is vacuumed or brushed along the direction of the pile during regular cleaniong. You may consider carpet rotation to compensate for shading.