Yellowing is a change toward a yellow appearance in a material or finish. It is a visual description, not a diagnosis of one cause.
What this means at your property
Paint, textiles, adhesives and plastics can discolor for different reasons. A stain following a leak or smoke event should be assessed alongside the material and history, rather than assumed to respond to a universal whitening product.
What should be checked before treatment?
Record when the change appeared, previous cleaning products and nearby moisture or smoke exposure. Ask about material compatibility and a test area where appropriate, and distinguish removable residue from a permanent change in the finish.