Regarding the black, as I understand it, apparently it helps reflect light back through the subsequent base coats etc to give your model's paint depth.... unfortunately I don't speak with first hand knowledge or experience here...
Perhaps a veteran can attest to this?
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In many cases yes, black does accentuate the primary color. Especially for light and reflective colors such as silver, metal, gold, etc.
Depending on how you (or if you) handbrush your first "prime" coat, it will streak, but the next coat will adhere it. And depending on the size of the PE part you are painting, try not to put too much paint on it and it will "blob" the detail making it look not as sharp as it is supposed to. Light (thickness) coats are the name of the game here.
As for metal barrels, same thing applies.
Jeff