Was it made clear that it had a partially invalidated warranty before purchase? If not, then I would simply argue that you cannot sell a vehicle with such a fundamental aspect of the purchase missing and not make it absolutely clear. They could never, ever get away with selling a car with it's primary engine and drivetrain warranty invalidated and not make it clear, so I don't see why they should try with paint!
This is incredibly poor service from the dealer. I would explain that you think they need to rectify as a gesture of goodwill, or you'll be suing them, reporting them to trading standards, Mitsubishi etc.
But they are obviously using the previous painted excuse as a get out cause, if I have to pay I do but if I can get them to pay by been stubborn and give a good argument then I am at least going to try.
My approach is to always try to be friendly, reasonable and understanding of their position, but to continue to request this be dealt with out of good will - after all, they might have margins, but you have a lot less money than the dealership does. Failing that, guns blazing, but not aggressive. Just state that you're being forced to sue them and report them, as they are leaving you with no other option.
If you don't want to go the full journey, maybe offer a deal - with you paying a portion of it.