May be worth a trip back to Halfords to try and get them to get a better match, but they would probably talk thereselves out of it. I bought some paint (5 litres) once and the team that mixed it missed out one of the main tints.
As you can guess the colour was nowhere near right. I did get another FOC and it was fine.
The Renault paint would probably be a better bet but if they get it wrong it starts to get expensive.
The problem with metalics is that you can change the tint of it by varying the amount of air pressure if using a spraygun. Obviously with a aerosol this can't be done.
You may get a deeper colour though, by spraying a slightly heavier coat from the aerosol, but then you get the risk of the paint running.
Another thing that has just crossed my mind. What colour primer are you using. This can alter the colour of the finnished job. Sorry for the long winded post but paint problems, without actually seeing the result is quite difficult.
Hope you get it sorted. Kind regards Oilyrag.