If I were you, I would message the person to say you were sorry they were disappointed but you would refund in full on return of the item and you were including a postal label for them to use for the return.

That won't necessarily stop a buyer giving you a neg or a neutral, it depends on how reasonable the buyer is. I am trying to project myself into the buyer's shoes here. If you refunded politely and quickly, I probably would not give a neg but I might feel like giving a neutral because the discolouration wasn't mentioned in the ad.

You can't rely on photos to tell the whole story as we consumers are always being told colours can vary from monitor to monitor and for all a buyer knows, a slight change in shade might be because of sunlight through a window or the angle a photo is taken on etc

Any fault, always mention. Sure, buyers may not read it but if they complain you then have a perfect comeback by saying it was prominently mentioned in the ad as well as shown in the photos. It could give you a basis then to fight any 'not as described' claim in the future.