This information comes from Lars Grimsrud ...
The BACK side of the steel GM Rally Wheels (both Chevy and Pontiac) and the rim/bead area was not painted silver.
Rather, the entire back of the wheel and the bead area was painted an odd light grey color. This color is perfectly matched
with the following Delstar mixing formula. This is the tip-top finishing touch for a set of perfectly restored Rally Wheels:

Lars’ GM Wheel Back Color, PPG Delstar (No color code)
Mixing Formula (1 qt) (cumulative units)
Color Units (cumulative)
DMR 475 6
DMR 490 106
DMR 400 272
DMR 487 442
DMR 415 447
DMR 476 457
DMR 491 477
DXR 495 497
DMR 499 1000
Mix the Delstar 8:6:1 Delstar/DTR601 Reducer/DXR80 Catalyst and shoot it with a paint gun..

Or: Ditzler 1977 Ford Dove Gray #2847
(This color is a close match)

Cheers Glenn

John,

I still have one of the Rally wheels that came off my '68 Z24 (it was the spare). Just looked at it out in my shed...it's the same silver/gray color on both sides.

Bill

John , I have had many "original" 15x6 rally wheels off of parts cars , and some were black on the backside. However some were also light Grey color from factory . Perhaps it depends on the year ?
I did mine with the black on the inside with Argent Silver on the outer side , and let the silver overspray on to the black .

Here is an old link stating that the insides are painted black ( for 67 corvette). ( its the last comment on the thread)
https://www.corvetteforum.com/forums/c3-tech-performance/952347-correct-paint-color-for-rally-wheels.html

Please help guys . The paint shop needs to know soon . Thanks