I would postpone it for two reasons: one, a slightly increased chance of Guiliain-Barre Syndrome (an immune over-reaction that, since its exact causes are poorly understood, is suspected of sometimes being triggered by getting the immunization while your immune system is already in an activated state fighting the cold -- though this has not been proven); and two, if you give your cold to the person giving the immunizations (or the receptionist or anyone else you interact with on the way), they could spread it to many more people!
The second reason is way more significant in my opinion.
But, if the flu shot scheduled for tomorrow is a one-time chance (such as a vaccination day at your work place) that cannot be rescheduled,