Are you sure it's faded? I'd have thought more likely dust trapped in the material. Perhaps just a slightly damp mf to clean it?
That was my first thought when I saw the OP’s picture. Such localised fading near the base of the door pull next to the window switches is unlikely IMO.
If it’s not dust, the it could possibly be the result of the driver’s hand regularly making contact with that part of the armrest; natural oils being secreted from the skin or some form of hand cream / moisturiser being rubbed onto the fabric over time on that part of armrest maybe?
@MrJollygood; If a damp microfibre doesn’t work, maybe try a steam cleaner if you have one or a proprietary car upholstery cleaner.