Since you didn't mention what year --- if it is mid 30 through 1950 you can bet it was body color.
I believe that it wasn't until the Crestliner in 1950 that the engine compartment barrier was painted a different color.
All I know is my '52 F3 has the firewall painted the same blue the rest of the cab is, that being said the truck is 65 years old so it could have been repainted... (I doubt it very much)
32 was black, body color for 33 on to ? Since the 32 firewall was removable, it was painted separate from the body and those firewalls were all black, as was the frame and fenders.