^^ what he said
As far as I'm aware, most places seem to mix paint on demand these days rather than keeping a stockpile of premix colours. Even Bunnings do it with house paint

A computer normally takes care of how much of each colour to mix in so you should be pretty right with colour matching.
The downside of that? You might need to buy a fair amount of paint to make mixing it possible/worth their while, unless they've recently painted a car your colour and have some left over. If you have any panels you want resprayed, get it done and ask them to put aside a little bit of the colour for touchups later. You'll only need a few mL so it shouldn't be a drama.