Phenolic yellowing is an unusual name and phenomenon. It happens a lot with rugs and carpets. I will try to explain this in most simple way possible. This explanation will shed a light on lot of doubts, hopefully!
Most of the people confuse it with the original carpet colour being retained under it. Yellowing can occur with carpets of any colour. The primary reason for this is Off Gassing. In the case of carpets, a chemical known as Butylhydroxyltoluene is used as a preservative. It is used either within the carpet on or latex of tufted carpets and in the under lays also. When a rug is placed on a carpet, it traps air and ceases the carpet from breathing. Then the chemical would float in the atmosphere and it reacts with the air and discolours the fibre of carpet. If a carpet does not contain BHT or if the carpet is not a tufted pile, even then yellowing can be possible.
Secondary backed latex glue back of the carpet may be the reason for yellowing. Smooth textured backing will have no resemblance to pile of rug. Basically, the Off Gassing trying to escape from the rug is being caught by the carpet is the reason for this yellowing action. It can never be cleaned. It is permanent.
Technical definition of Phenolic yellowing
This yellowing is caused due to the presence of Phenolic compounds in the textile material and it reacts with oxides of nitrogen in an alkaline medium. This phenomenon is associated with storage of finished textile material. They are packed in polyethylene materials or cardboard cartons. Aromatic amines (Para phenylene di- Amine) and Phenolic compounds (Butylated Hydroxyl toluene) are used as anti oxidants and stabilizers in organic polymer packaging materials and in foams and lubricants. There are Phenolic derivatives from lignin in cardboard. They form yellowing precursors