From what I understand, chickens don't get 'colds' like people do. Most respiratory symptoms in chickens will end with the chicken being a 'carrier' and infecting others possibly... if the chicken survives. Additionally, if it's viral (not bacterial), antibiotics would only help with any secondary infections due to that. A lot of people will cull any birds that display respiratory symptoms.
Having said that, if you decide to treat it... what you use kinda of depends on what the chicken may have contracted. I had respiratory symptoms in a brand new flock I got, and decided to treat them rather than cull them (and then have a closed flock-- nobody in/nobody out/no hatching eggs). First, I used Aureomycin powder mixed in the water. There was some improvement...but lingering issues that just didn't quite go away. I next tried Sulmet-- no change in three days, discontinued use. Next I tried Tylan 50 inject-able BUT I gave the dose ORALLY. This was what worked for whatever my chickens have. Noticeable improvement in 24 hours, nearly 100% by day 3. Whatever it was, was sensitive to the Tylan 50 but not the other meds.
My pullets symptoms were listlessness, a 'wet' sneeze', and two had one eye each that , initially, was stuck shut but NOT swollen. When open, it was watery. (not running/dripping...not foamy or bubbly, just like a 'tear' was in the eye) The affected ones also had a small amount of clear, water like nasal discharge occasionally.
The first meds I tried gave them back some energy, stopped the eye/nasal watering (but one still held it's eye shut, not stuck, just closed) but the affected ones still sneezed, even after nearly two weeks on the meds. The Tylan 50 injectable, given orally, is the only thing that's returned them to 100%.
HOWEVER, it might NOT be what's best for whatever is infecting your bird. Know that the chances that if your bird recovers, it will be a carrier of the illness is great. This means it can infect new birds, it may pass it on to the eggs (fine to eat, but chicks may become ill), and it may 'relapse' and become sick again with the same thing if under stress, etc. This is why many people cull them. Just depends on what, exactly, is affecting your birds.