As promised, here are the colour photos. The last (B&W) shot, is from the SS 'Walk Around' book, a NAA photo, and is shown below the photo stating that the tank is bare metal, and the firewall yellow ZC. In this photo, there is a definite tonal difference between the surrounding bare metal areas, and the engine bearers, and I'd suggest yellow ZC also. Note that clearly seen in the P-51B photo, the firewall is green ZC, or Interior Green, more likely the former.
I think it would be unusual to have an important vessel such as the oil tank left in bare metal, rather than some form of protective coating, and the three colour shots support this, but, as with many things, it's possible there were exceptions.