That is bollocks, they should have sprayed it right first time not blame the insurance company for not paying, they already have the paint and lacquer they just need to paint it again properly as it won't cost anymore in materials just their time, but as they did it wrong anyway so what if they have to do it again properly.
The fact you signed the acceptance form (or your wife did) is another excuse for a cop-out because at the end of the day they have not done a competent job.
I can't help with the legal aspect but how can the spray shop not match the colour? even if it has been resprayed a completely different colour they have the technology now to match almost any conceivable colour using computers.
We are talking about ONE easy standard ARG colour and metallics are easier to match than straight colours but you have to paint across the adjacent panel to blend in the colour correctly, this is a know fact in all car repair shops and they all will need to do this at some time. Obviously your spray shop has sprayed upto the next panel and thought the colour would match without checking first and now can't be arsed, believe me it can be done but sometimes you have to put in more effort to end up with the correct colour match.
If a car spray shop can't match a simple colour then they should NOT be in business.
If the back-end was resprayed before and you have not noticed then it is probably because someone has taken the time to match the colour correctly, in either case your spray shop obviously have not bothered to match the colour to the rest of the body.
You don't need a respray to match a colour especially if it is an easy to do one shade no variant metallic as we have already deduced.
The spray shop is incompetent and merely suggesting that the back end has been resprayed is just a cop-out.
Basically all they need to do now is respray the area they cocked up but now need to blend to the next panel, which is what they should have done the first time.
An example would be to match a damaged boot, the insurance company will not authorise the cost of spraying adjacent panels because they are not damaged and the paint might match perfectly anyway and there then would be no need to paint the wing tops to match the colour, however this responsibility lies with the sprayed who should factor in having to spray the wings to end up with a colour matching the rest of the car. This is normal practice on any attempt to colour match correctly and we're only talking about a small car with a tiny boot lid and even smaller quarter wings.
Stephen.