As a kid, I built a lot of model cars and had a large selection of Testors model paints.
Is that still a good option for touching up plastic building models?
I still have a quite a few (perhaps 20 or so) Testors model paints and they still work very well. However, as with most, if not all model paints, they have a certain luster to them, even the flat paints, that will not match the original model. The solution to that is, after you're done painting, to give the model a coat of a clear flat finish. Testors Dullcote worked well for this but it has been discontinued however, I have also used a flat finish from Tamiya. Hope this helps.
Krylon clear matte finish in a spray can works well. And you get almost 3 times as much for less than a can of Dullcote sold for.
Len