as far as using red primer goes, that stuff went the way of drum brakes. red oxide was most likely lacquer, and most (make that all) reputable body shops use urethanes, and epoxy primers nowadays. dupont has a system called valueshade, whereas all colors can be undercoated with 7 shades of grey, ranging from a greyish white, to an almost black. the valueshade for sonic blue is #6, so it is not the darkest, but almost. the best thing for you to do, if you plan on painting directly over this primer, is go to a bodyshop supply house, give them the paint code you plan on using,( or one that is close) and find out what the valueshade is. have your primer mixed as close as you can to this value shade, and go from there.