You guys realize it would require more effort and logistics problems than it would be worth to save .07 cents per engine. It's floorspace (that is precious real estate) sequencing (you pay someone to do that) build complexity (savings for 1000 cars lost if the engine gets installed in a retail car; someone gets paid to swap it) and an engineering corrosion buy off (thin parts like valve covers would never pass that spec; engineers don't work free) and service warehouse inventory. It's even a wasted line of text you pay someone to enter it into a parts catalog.
I do believe paint may have varied in quality or application across the line, so you may have seen those engines.
They started painting all engines black in '82, that was the cost-save.
Related tangent: (2017) Demon engines were originally planned to be blue. They used red because they didn't have a blue that passed the corrosion spec. That's not inside info, it's part of the press release material.