My understanding of the mfg process suggests that it would cost more to "one off" a vehicle than it would to just take one off the line.
Most of the "generic fleet vehicles" i've seen come in plain white, with minimal glass, and no, or very basic radios.
The idea of shipping a new vehicle in just primer seems a little silly, since as Red mentioned, that would leave everything under the hood in primer.
I'd guess if you were ordering enough of them, GM would be happy to paint them dayglow orange with purple spots right there at the factory for you.
