You might want to try something like this
http://www.rockler.com/product.cfm?page=17830&filter=touch%20up%20spray%20gun, then you can get color paint mixed at any automotive paint store. The glass jar holds the paint and the compressed gas creates the "spray can" effect. You may need to add thinner to get the best result, but just a little experimentation should do the trick.
I must confess that I have no used one of these, but I have considered buying one.
If the touch up is really small, you could consider getting an air brush paint kit. I picked one up from Harbor Freight pretty cheaply and it does what I needed it to do. You could power it from an air tank for quite a while.
Just random thoughts to give you more to consider.
Charlie G