Maybe try Transtar water based primer in black. It is tough stuff. I was working on a small engine really close to a 67 Mustang that a restoed a few years back. I splattered the whole front end with fuel and oil. I thought it would kill the paint. It just wiped off with prep solve and I was able to paint over the top. Plus it looked cool.
I agree with what's been stated...basically, do your best to get it right so the color doesn't matter as much....but white really does hide the imperfections pretty well. I was lucky that I always intended to paint my car white, and this was the first body work and paint I had ever done. If I had tried to spray any other color, I may be ashamed to let anyone see it. It was FAR from perfect, but not bad for the first car I ever painted.
Because of the "place" where you welded on the new "skins", you will no doubt be Covering the quarter panel from "front to back" and "upper body "roll" to "body line" completely with Bondo. You Can use a 8" rotary sander to cut it all down to get an "Approximate SHAPE". Use a straight edge to SEE where your low spots are. ( the 1/4 panel Is NOT straight, but this will give you an Idea of where to ad) Then re-coat with thin layers of bondo, and start using either a "Air powered "Inline sander", or Hand held "Board sander" (about 3" x 20"). I've been doing it for 40 years. Red is absolutely the Most "Expensive Paint" you will buy. Flat black does not "Show" alot. if you are going to all the trouble to begin with, spend more TIME and "do it right." use Urethane primer, NOT Laquer.
You should repair some smaller dents first using a block so you can get the idea of blocking. Do not just sand on the filler or you will dig to much out. You want to sand on the metal and work your way across the filler. Doing smaller dents first will give you the idea of when to stop when your filler feather edges. You should be able to rub your hand across your work and just feel a smooth flat surface with no edges and no high or low spots. Light colors are better for hiding imperfections in body work than dark colors. Bright whites are easier to lay runs in well lit paint booths because the brightness of the lights and brightness of the bright white color makes it hard to see your paint going on after the first coat.
also, When you "Think" you have it sanded to the "correct contour", Take a Clean Rag in the palm Of your Hand, and slide it over your work. It will show you "IF" you are even "close". If there is ANY low/high spots, you will Feel them this way.