I'm about to start on Tamiya's 1/48 A-1J Skyraider (61073) and I have a couple questions in regard to painting. I've attached a PDF of the instructions here

https://www.tamiyausa.com/pdf/manuals/61073ml.pdf

1. I have seen part D9 painted many different colors, from the underside white or light gray, to bare metal, or even the engine crankcase's dark gray. What was the purpose of this part? Was it painted the same color as the engine crankcase from the factory?

2. Did part D10 stay attached to the propeller when the propeller was removed? If so, was D10 the same color as the prop hub, or was it dark gray like the engine crankcase?

3. What was the proper color of the rear side of the pilot's head armor plate (A20) and fuselage decking under the canopy? Was it painted the cockpit interior color, or the exterior camouflage color?

4. Did the color of the engine dive flaps and interior of the engine cowling (D4, D6) depend on the camouflage color of the plane?

D9 is the engine reduction gearbox cover; it should be painted engine grey, the same colour as the crankcase. I'm not certain, but it appears that D10 is part of the propeller hub assembly, and would remain with the hub when the propeller is removed.

The back of the armour plate would likely be the same colour as the interior, but since most of it is covered by the canvas shroud on the headrest support, the question is rather moot. The interior of the engine cowl looks pretty grimy and blackish: