IMO, anything mixed in a can off the code is probably not going to match very well.
I would go to a local automotive paint store. They have a device they put up to your paint that tries to give an accurate color for them to match. It likely still won't be perfect, but it should be closer.
Whether or not you need a spray can comes down to how big of an area you are attempting to touch up. If you have a larger section of damaged or flaking paint, then you will need the spray can. If you are touching up rock chips, then a paint pen should work well.