The hinges and the springs were gray phosphate, not painted.

The factory dusted the springs and hinges with phosphate for a reason. Once a little grease was applied at the factory, it would sink into the phosphate coating, and then the assembly would never need to be greased or lubed again. Standard practice for all GM cars.

Over the decades, thousands of people painted their springs or hinges, usually when their engine(s) were pulled, and they wanted to clean up the engine compartment a little. The gray phosphate coating process wasn't widely available in the resto hobby until the 1990's, and even then, only well-equipped resto shops were doing it, and it was fairly expensive compared to rattle can black. Probably thousands of cars were restored or partially restored in the 80's and 90's in an owners' garage, and with little information available (assembly manuals, web, etc.), well, when it doubt, paint it black.

The same gray phosphate coating was used on the hood catch assembly, and like the hinges, GM did this so the assembly would never need to be greased or lubed again.