I just bough cheapish paint from halfords using a plastic grey primer and their version of Titanium Silver. I didn't see any reason to buy a proper BMW can, because the match doesn't have to be exact as the car is space grey.
First two pics are in primer and painted


These are before and after on the car.


I actually bought the grills from ebay for a tenner so still have my originals if I need to change back.
What do you think?