yeah, i've had black rats that went almost completely chocolate on me when they got older. though spider seems to be going grey instead so go figure. coat coloring changing isn't uncommon at all and doesn't normally pose any problems, well unless you were showing her or something but that'd be about it. i had a young rat who's but turned brown from pitch black about 5-6 months. i thought it was just the sun lighting up her hair as she would bask in it for hours but then she got old and turned brown (mostly) so it could have been that too. in any case don't worry about it, its normal.