First of all, chickens do not get "colds". The seller should know this. However, if she was soaked to the bone, she could get pneumonia. That does happen and is the one time I would be comfortable treating with Tylan50 injections. BUT, since you really don't know if she already had something when you got her (chicken diseases most of the time leave them carriers, like Typhoid Mary, since many are similar to herpes viruses), she could have already been a carrier. In other words, they don't really recover from things like Chronic Respiratory Disease (CRD)/Mycoplasmosis or Coryza.
It's a hard call. I would never treat for contagious respiratory disease, but I would treat pneumonia. You'd have to already have had the bird and known her history to know which was the case here. And since most ask at this point what I do for contagions, I would separate the symptomatic bird immediately and cull (euthanize) to save the rest of the flock.