My experience is more limited than most, but from what I've seen lost color could just be stress. The internal bleeding's not a good thing, but if I had to guess, I'd bet on a fight happening. Guppies aren't really schoolers so much as "What are you looking at, is it food?" so depending on if anyone in there is agressive that might explain it (Though for guppies that would be playing awful ruff. Do you have other fish in the same tank, or is it specie only.). Is anyone else showing similar symptoms, or showing signs of fighting (Like nipped fins). Also, do you have a quarantine tank, or anything like that, you could slip her into, just in case?