As part of my restoration project of my COE, I took an autobody class at the local community college. My project was to restore my fenders. My truck is mostly rust colored but was originally black. Not sure that I want a very large black truck, so I painted the fenders Mariner Blue, to see if I would like it. As Tim said, it is impossible to tell from a photo that is displayed on a computer monitor, but I would guess that your firewall is faded Mariner Blue. The best way to tell is to get ahold of a paint chip and compare it directly to the best preserved and buffed area of your firewall. I am sitting here comparing your color to my paint chips in my '51 Truck Data Book. I really think Windsor Blue would be much lighter and have a tint of grey but only you will know for sure.
