
It's hard to judge her weight without seeing her, but when you stroke her side, can you feel her ribs fairly easily, or do you really have to poke through fat to feel them? And when she is standing and you look down on her, can you see a waist between her ribs and her hips? She seems to be at the low end of both the weight and height range, so she doesn't sound hugely fat. Perhaps she needs more exercise?
The only thing I can think of that would darken her colour slightly would be carrot, but you are already giving that - too many wouldn't be a good thing. She probably will never be darker than her ears are now.
Hope this helps :)