Everything in the cab was the same color except the steering column, steering wheel, column shifter and linkage were all a brown color. But a lot of people put those the same color as cab now. I believe the brown color was just like the brown on the reproduction horn button, made by GM.
Probably originally the paint coverage was not complete, like under the dash, but I would put paint on all bare surfaces.
Exterior color ends at the point the exterior skin ends, so that panel you mention would be interior color.