Original grilles were cast in both silver and black plastic, the body color was applied to the proper base colored grille. There were 4 different grilles, black and silver non RS grilles, and black and silver RS grilles. The grill sections were never painted on grilles from the factory. GM eventually dropped the silver cast grilles from their service parts and only retained the 2 black ones, so after that point all SS, and Z28 standard grilles and RS grilles going on Black cars had to have the center sections painted silver, along with the body color around the perimeter.
Don't know what kind of rack the grilles were on when they got painted, but there had to be a center mask that fit over the grille sections to keep overspray off the grilles. No one masked each grill with tape.