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