Chickens dont get colds, they get a specific disease(s.) Or, it could possibly be an environmental issue such as dust from feed, ammonia fumes from soiled bedding, pollen, smoke etc...causing your chicken to sneeze. The enviromental issues can be eliminated. Here's a link to respiratory diseases. Scroll down to "infectious Coryza" and Mycoplasma Gallisepticum" (MG.) It seems they are the two most prevelent diseases floating around. http://edis.ifas.ufl.edu/ps044
I am not sure what it could be BUT we recently found a product called Oxine. It sounds like it could be an amazing product. There are several ways to use it, a fogger (which seems to be more than most people are willing to use) or it can be administered directly into their water. We have ordered some but have not gotten to use it yet, we have just been too busy! Look into it though. It sound like something that can be used as a preventative and also as a treatment.