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