Bill,
I am by no means versed in how those choices were made in the later '60s, but from what I've researched through the early sixties, there was no distinction what-so-ever between primer choice and final top coat color. The plants simply had one batch they used for everything until a new batch came in and they switched. To keep light with light and dark with dark, they'd have to have two primer lines, then separate the cars to each line. I'd think that would be too much work.
IF there's one guy I know who could answer this most accurately, it would John Hinckley over on CRG. He was there.
Verne
PS: Or you might even try asking MoParts Dad, who has addressed many of those details from my queries on my '63 Impala anniversary gold thread. Not surehe worked in the Camaro or Nova plants though....................Mo?