How long has it been since you changed her food? Before/After spay? I'm not an expert but it doesn't sound like anything's wrong. Is she eating and drinking normally? When she's awake, is she alert and playful? Are her gums moist and pink? If answers are yes, than she should be ok and darkness might be from the food. But just to be sure, I'd like to suggest you give your vet a call and ask. Never hurts to ask questions.
Also, have you checked the posts on dog food? I believe there are better quality dog foods compared to RC and Pedigree. I feed my two Blue Buffalo Life Protection for small breed.