Would they've used red lead primer on the chassis and instead of wasting money painting all the bits that can't be seen just painted the outside black? Just a hypothesis mind you but railway companies were renowned for being tighter than a ducks arse.
The locos left over the accessible pit in the NRM at York all seem to have been painted a lighter colour inside the frames whatever company they came from. Was this to give better vision to the driver carrying out routine oiling etc, or for the fitter carrying out repairs, perhaps?
On the outside, the black would hide the grease, ash and other dirt that would get on the frames and so keep the loco looking tidy even if it wasn't on close inspection.
In my experience the frames would be outsourced and delivered to the assemblty facility in an etch primer (red,yellow,grey),same as some lorry frames etc,they would then assemble the loco,and then spray paint the livery,so only the outside of the bogies would be sprayed black etc and the body work in whatever colour,and the undersides are left the etch primder colour.Later on in life when loco's are revamped etc they usually steam clean the undersides and spray everything the same colour,black,or whatever takes their fancy,I have even seen white!!The engine bay gets the same treatment and everything gets sprayed including fan belts,batteries,everything,just to make them look new/revamped,also water hoses and evrything gets the blow over,after a few hours of running you get the paint burning of the manifolds and cracking on hoses and cables and it looks a right mess!!
I bet it smelled lovely too! Bad enough the old painted mild steel exhausts we used to fit in days of old. Used to frighten the customers to death the sight of all that paint smoke coming from under the car.
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