What is meant by car primer is Halfords Plastic Primer not the grey stuff used to prime the bodywork. It is formulated for use on the many plastic parts found on modern cars, bumpers, wing mirrors, etc. The older traditional types of paint often do not adhere very well to some of these plastics which is why you need to use HPP on these parts. After that you can spray on normal car touch-up paint. HPP comes in the usual primer grey, red oxide and white colours. It is a very fine primer and you can use both enamels or acrylics for the top coat. As it comes out of the can with some force it is probably better to decant it into a jar and then put it though your airbrush. If it starts to thicken it can be thinned with lacquer thinners. It is my preferred primer and I have never had any problems with it.
There is also a filler primer with is yellow. It will fill tiny scratches from sanding and even reduce deep panel lines. Usually you will sand it after application to get a smooth finish. Useful for getting the prefect finish needed for metallic finishes.