Even I can match Vauxhall white
White, assuming its not faded into cream, and Black (Plain Black) are probably to 2 colours that are most forgiving with Rattle Cans. in fact most Non Metallic colours work well, although Red can be an arse if its been Sun bleached.
Once you mention the word "Metallic" its starts to tricky, as you trying to match the Metallic reflection it gives off as well as the Spray Can pattern, having said that Silver is not a bad Metallic to spray if its done sparingly, bit by bit with some wash in (over spray) into the existing panel.
But as you say Al, its all about Prep Work, then more Prep, and getting the Clear Coat to look good, if at the end of the day its not going to happen, best bet is do all the Prep and Primer work done yourself and then get some quotes for a Pro just to blow it over in the Base Colour and Top Coat.