Chances are there were different length screws securing that panel. When you reinstalled it, you may have used a longer screw than was removed from that location. I would suggest removing the panel again to verify what the screw apparently punctured. Refrigerant is colorless and I have not seen red colored refrigerant oil either. It might be a dye that was installed in the A/C for leak detection.
Coolant could be red, if the screw hit heater components. If leak is coolant, you will need that plugged up before travels.
Last thing I thought of being red, is ATF fluid used for hydraulic systems. Any chance there could be a hose back there? I would want to be sure of what leaked and from where.