The next time I do some paint testing I want to try lightening the PS Pacemaker Red with Golden High Flow Naphthol Red Light. The latter is a bit transparent but it is much closer to light barn red. As you observe, adding white creates “Mary Kay” pink that would be fine if you are modeling the GW GP7 #705 in the WP Museum. Also the PS Pacemaker Red is a bit on the magenta side. Perhaps a few drops of a bright yellow would help here. A cool red can clash with a warm gray. So far the Golden acrylics seem compatible with PS paint.
My interest in black primer is only for the trucks. The gray primer works great on the car body to create a uniform neutral base color.
On trucks I want to use a primer for better paint adhesion because during ops they will get knocked around more than the car body. Black seems useful because it would disguise any spots that receive too little primary color and weathering. A lot of O-scale brass trucks, including the ones on the C425, actually equalize so I want to minimize the thickness of paint to avoid gumming up the works.
The NR&W hoppers will be black. About half are currently black so I just need to remove the pad printing then shoot a light black coat on the sides and ends. The other half are a box car brownish red.
I’m not a fan of rattle-cans, but Rick uses them with great skill. He warms the can in a pot of water (not boiling!) to make the paint more fluid. I prefer the greater control afforded by an air brush.