Some years ago, I had a '72 GS Convertible that was a "special paint" car. Definitely painted after assembly; front clip was NOT a perfect color match and had overspray everywhere, but THIS WAS ORIGINAL PAINT.
Color mismatch was pretty common, as in most cases the front clip was painted in a different part of the plant (or in a different plant) than the body.
In the case of Buicks in Flint the front clip was painted in a different part of town
then the rest of the car (bodies were built on the north side of Flint and trucked to the final assembly plant).