Suggestions on Primer Color

[custom10nut]

I’m looking for suggestions/advice from the painting experts on the forum.
My plan is to repaint my truck the the Original Color scheme which is Medium Olive /White.
What would be the best color of Primer to spray under the one step color coat, to get the best depth/appearance out of the color. It’s been a long time since I did any serious painting, so I’m researching the new technology out there.
Back in Day I was taught by a Master on how to paint Imron and Candy Colors.
Right now, the truck is pretty much factory Black primer and some places that are the red colored primer.
To make sure I end up with a uniform finish, I’m priming to whole truck, no piecemeal colors.
I’m not such a perfectionist that I expect a Show winning finish, but I want to have something that looks good rolling down the road.
So I guess the final Color question is :
Black?
Light Gray?
Dark Gray?
Red/Rust?
Green?
I’m just hoping for some insight