I took one of the front wheels off my bota, brought it into my local auto paint supplier and he scanned the inner rim with some sorta light pen connected to his computer (on a nice clean painted area, unaffected by UV). He was able to mix up as many rattle cans of the paint as I wanted for not much more than the cost of an off the shelf rattle can! Now I have the code, I can just order up what I need when I need it. For touch up, a rattle can is the way to go.
It sprays on good and matches the existing paint very well. Not as glossy as a professional spray painter would apply, but.... its a tractor.
And my wife said its a good match, so if it gets passed my wife and her "keen eye" for colour coordination, then its good.