I own the same pair and early on I saw a spot on one of my pads. I assumed the same thing you did...it had gotten dirty. I mildly wiped it with a clean, damp paper towel and saw slight discoloration and quickly concluded that the dye they use on the light tan/brown ones does not last. So I left them alone. I figured they would not stay beautiful long. I use mine regularly, but not as much as you. Fortunately, mine still look pretty new. You have confirmed what I feared, though, they may not stay new looking for long.
I would definitely make a run at B&O on a warranty claim and see if you get any joy on replacement pads. I would send them the pictures you posted. Let us know how the service reponse is, please. More of us may end up in the same boat...
HS