Tricia, I've had my Flower for about 2 1/2 years now, and in the time, his
coat has gone from dark browns and tans and creams to almost exclusive
cream.  He looks like a DEW except he's still got some of his original
color on his tail and just a smattering of darker hairs along his
shoulders.  And while the others haven't had so drastic a change, Boobah
has gotten lighter and Buttercup has gotten darker in the 3 1/2 years I've
had them.  So while I'm not an expert, my experience is that a color change
is perfectly normal.
 
Wendy and House Penferret: Boobah, Buttercup, Pica, Flower, and Kibble
[Posted in FML issue 3302]