professional carpet cleaners call “off gassing.”
First thing you should know is not to confuse this with the original carpet color being retained while a rug was laid on top of it. This phenolic yellowing can happen on any carpet color because of the BHT (butylhydroxytoluene), a preservative that is used within the carpet. This is especially true for tufted carpet with latex and the underlay.
When you place a rug on your carpeting, it is the same as sealing in air which causes the carpet not to “breathe.” As such, this trapped air contains the chemical that usually floats out into the atmosphere and instead, it causes a reaction that discolors the fibers.
However, not all carpet has a tufted pile, which means it doesn’t have BHT that still experiences phenolic yellowing, and then the rug may be the problem with a secondary backed latex glued backing. Instead of air being trapped in the carpet, the “off gassing” isn’t able to escape out of the rug, thus the phenolic yellowing.