Is the collar yellow? It could be dye rubbing off on the fur.
I mostly see it with red collars, after wearing them for an extended period of time the dogs sometimes get a red tinge to the fur around their neck.
Cheap leather from China no doubt. Order a collar made in the USA with guarantee that dye won't run. I got a collar on the internet for $50.00 with American made products and didn't have a problem. The cheapie one from PetSmart ran red that was terrible. Dye isn't good for the dogs, either.