Primer for PR4 (red '95)

Just fixing some rust spots. Not looking for perfection, but I used red oxide primer figuring it would be a good base for my red top-coat. Anyway, a coat or two later and the red oxide seems to still be showing through... Even over silver POR-15 the red doesn't cover that well (I sanded the POR with 400 grit).

I guess a very light (white?) primer would be best? Is there primer colour that is best for red.. I dunno yellow or something?

Thanks...

I've found it's more important to have one color primer or base color when painting. The color of the primer doesn't really make much difference if it's all the same. Painting over light spots and dark spots, will make the finished product splotchy. Feathering the edges of the primer patch helps some.
I've never had Krylon primer grey or brown lift on me. Rustolium rusty metal primer also works well, but has a long drying time. I've had poor results with the Rustolium grey automotive primer.
The color of the primer, especially when buying automotive primer in cans, is often keyed to the function of the primer. Yellow is easily sanded, grey covers well and brown is a sealer and a little harder to sand. Guys often put down a coat of sealer primer, then spray a light coat of grey over it, if the top coat is light.
Primers can give you trouble, some paints lift certain primers. Making sure it's dry, dry dry, helps avoid catastrophe.