It's called 'rusting' and is completely normal. A lot of dark, solid (non-agouti) rats will rust with age. I had a mink girl who rusted in big patches all over her back as she got older. It's genetic. Some rats 'silver' (get white hairs) and some rust. Most breeders try to breed away from that to get more vibrant colors that stay true longer. Either way, calm down and don't worry, it's no big deal. 
