Chickens do not get "colds". They get respiratory diseases or infections, some of which can be quite serious. You said that your chicks have sneezed ever since you got them; what type of bedding are you using? If using cedar woodshavings or dusty sawdust, that could explain the sneezing.
However, you are now noticing droopiness. This is a sign that it may be a respiratory disease, not something from the enviroment. The green droppings are occurring because your chickens are not eating enough, anther sign of sickness.
Have you noticed any blood in the droppings? Lethargy is one symptom of Coccidiosis. The medicated feed that you have been feeding them is supposed to aid in the prevention of this disease, but does not always. If you notice more symptoms like bloody diarrhea or sleepiness, it might be Coccidiosis.
As of now, it seems like your birds have a respiratory disease. I'd get some Oxytetracycline (like Tetroxy HCA-280, LA200, Terramycin, or Duramycin). Oxytetracyline can work in curing a respiratory disease, unless it is viral.