Unfortunately, the picture isn't quite clear enough for me to make out the nail. It's possible she has pulled the nail out of its nail bed a little way, which can be quite sore. If she isn't limping, I'd give it a week or so and then attempt to gently look at the nail again and go from there. If someone helps hold her, will she let you get a better look? It is normal for dogs to have different coloured nails, so it's less the colour and more her indicating discomfort, which is the concern here, IMO. If the nails weren't kept short in her lifetime, the nail itself may also have become ingrown.