We've seen this same thing with our ferrets.  It seems like it may come
from their skin oils.  We used to keep harnesses with bells on our
fuzzies(to find them easier) and after awhile they began toget an orange
tint under the collar.  It occurred to us that maybe their skin oils built
up under the collar.  After they quit wearing their collar, and had a few
baths, the orange began to fade away.  Also, if you have any white areas
on their bedding, the same color will be there after some time.
 
This just seems to be what we've come to find.
 
Take care,
Jen, Mike & the fuzzies - Loki, Fria & Jamocha
[Posted in FML issue 3213]