Keiko's old collar, the black one, did leave a small portion of her neck right in front a dingy gray color. I don't think it was the dye wearing off though, b/c she had that collar for years, I believe 6 to be exact, and I would think the dye wouldn't keep coming off after all that time.
I actually think the marks were made from either her tags or the metal o-ring on the collar. Okay, I just made a quick check and she still has a small mark, but I think it's from her old collar and the fact she hasn't had a bath in a long time. The mark isn't nearly as dark or noticable as it used to be, but it's a buggar to get to come out.