touch up is only really for really small areas. Anything from accident damage needs to be done by a pro, otherwise you'll have a noticeable colour mismatch.
Each batch of a given paint at the factory has a slightly different colour, hence two cars of the same colour will actually be very slightly different colours, and as far as i'm aware any professional bodyshop will mix the paint as close to the original paint (by using cameras and a computer to scan the existing paint), and SMART repairers use some methods such as blending (depending on the type of damage) to get a visually-correct repair. This is what people refer to when they say things like "Its a good colour match".
The touch up paint that you get off-the-shelf is only ok for the odd stone chip here or there, because its just a generic non-batch-specific small pot of paint.