Apparently decongestants (which are in most cold remedies) can be dangerous for diabetics, with the potential to cause heart rhythm disturbance. Paracetamol or ibuprofen of course wear off every 4 hours. My pharmacist suggested a once-daily antihistamine in addition to paracetamol or ibuprofen, it seems to help cover the time when the painkillers wear off. Of course check with your doctor/pharmacist first that its okay for you to take antihistamines.
I find my bs/insulin requirements may not get back to baseline until a week after the cold seems to have finished.