I like black primer for red. Just graduating from rattle can's myself, so I'm far from knowledgeable.
But so far, the work we've been doing we've used a variety of primers - red, brown, white, gray, and black.
For red paint, I like the black the best. It's easy to see if you've missed anything. And as somebody else pointed out, when (not if) you get some nicks and dings, the black looks a lot more natural and it doesn't catch the eye - lighter color primers kind of scream out "I've been scratched".
If you went with red primer, I can almost guarantee you'll miss some areas and not notice it till well after you're done painting.
If the color of the primer shows through the paint, I'd think you're not putting enough paint on. At least for any paint you'd likely be using on a tractor.