One possible safe ferret dye is Kool-Aid, which I have used on kid's hair.
Take Kool-Aid powder concentrate (the kind with no sugar) and mix it with
SOME water (not the full amount) and paint it on with a brush.  It washes
out fairly easily after the event.  However, children don't LICK it off,
and I don't know how problematic concentrated dye chemicals might be to a
ferret!
 
Just a thought. - Erika
[Posted in FML issue 2841]