The only time I have ever had color transfer on a bag was from jeans on a white bag. I used baby wipes and the marks mostly came off.
I'm wondering if it could be that black fur vest that I wore under the moto... When we enter a building or cafe or restaurant, it would be sweltering inside and I would take off my moto. My bag strap would then be on the fur vest... Hmmm!! I wish there was a way to confirm this without actually getting more transfer on my bag!