Boris

I've built two red arrows hawks that came in red plastic, then primed them with grey before painting them red.

I was told by a professional painter long ago to always use a different color primer and/or undercoat to the top finish - it is the only way you can be sure you have a complete and even coverage with your top coat(s). The trick is to find something different enough, but not detrimental to the colour you want. Sometimes it even helps coverage, like using white undercoat with yellow topcoat.

Not a bad idea that, now if only I wasn't such a tight git using one primer for everything