If you are building from the AMT, Revell or Italeri (US trucks) range of kits - here are the "standard" colors the trucks were offered with, meaning stock order, not changed. This information is from sales order forms, brochures and spec sheets.
Frame paint was always gloss or low gloss (typical of the Detroit builders). Flat black or matte black was optional. Flats and mattes were not good for frame paint as the promoted debris/salt/moisture/snow to stick the paint and were difficult to clean.( Finding a 1969-1985 truck with an actual flat-black factory painted frame is pretty rare).
GMC Astro and General: Black. Optional color to match cab or accent. Optional matte black.
Chevrolet Titan and Bison: Same as GMC
Ford C - Black, optional matte black special order other color
Ford L Series - B, optional matte black, special order other color
Mack: Black, other colors optional
Diamond Reo: Black - special order other color
White: (Road Boss, Western Star, Autocar) Black. Other colors optional
White-Freightliner: Any color. If no color selected chassis was cab color.
Peterbilt: Any color. If no color selected chassis was cab color. In later 80's black sneaked in as the default chassis color. Matte and flat available on special order. (The factory hated flat, even for tops of hoods).
Kenworth: Any color. I no color selected chassis was cab color. In the 80's black sneaked in as the default chassis color.
International: Black. Other colors optional.
All of the north American truck manufacturers paint the entire chassis as a completed assembly. Frame rails, crossmembers, springs, axles, shocks, 5th wheel, etc are painted chassis color at the same time. Despite the model companies box-art paintings showing components in
separate accent colors.
None of the manufacturers offered unpainted frames. Some of the western builders (KW, Peterbilt White-Freightliner) offered "in primer" trucks - sent from the factory in primer so the customer could apply their own paint.
The big exception here was Diamond T - in the 60's they built several advertising trucks with aluminum frames and crossmembers that they polished and applied an anti-corrosion coating, painted the suspensions/axles etc silver, and used these trucks to show off all of the lightweight aluminum components.
The polished frames looked nifty-spiffy.
Once a truck left the dealer and into the hands of the customer - all bets are off and frame colors became a gold-rush wild-west free for all.