Comb tips turning purple is usually a symptom of anemia or lack of good oxygen flow. Was there any sound he was making when he was breathing, like a wheeze or crackle or such? Did you check them over for any external parasites? Parasites can bleed them dry and give them terrible anemia if not treated.
One thing about the antibiotics - if you started them, you should give them for a full 7 or 10 days, because shorter treatements are what causes superbugs that are immune to antibiotics. Keep the treatment going!
Other things that you should think about: do you have an avian vet or a vet who will look at chickens nearby? It would be a good idea to have them test a fecal sample to rule out internal parasites. They may even be able to identify what is going on. Thing is, most vets around me think of chickens as uncooked meals, not pets or livestock to be taken care of, so I have had to learn to do most of my own vet work.