Tim,
Do you have any info on the "Sheen Gray Metallic" mentioned in the Sept. '53 Service News as the '50-'53 interior color?
Also, you mentioned Shell White. This was originally a '53 exterior color, but it was replaced early in production by Pure White (you've probably seen this in the Feb. '53 Service News), though the '53 resto pack still lists Shell as a striping color. One of my sources says Shell White was the exclusive upper color on '54 two-tone exteriors. Maybe the two-tone RPO 438 became available in April, '54?
I have a '48 sales brochure with a rendering that sort of shows the engine compartment of a red truck. The inner fenders and the upper radiator baffle appear black, but the firewall is not body color (red). It looks tan-ish. This could be another one of those artist inaccuracies. Though I wonder if all of the assembly plants followed the same painting "rules".