I was at the York outlet a couple of weeks ago (and accidentally bought a bag), and when the SA produced the dustbag for it, it was a white one. I asked him why it was white and not brown (as the bag I was buying wouldn't have had a problem with colour transfer) and he said something about Mulberry using white dustbags at the outlets and brown ones for full price items as a way of differentiating them. I'm not sure if this is true or just what he was told, but it could be the reason?