At Homecoming you get to see how the BMW final inspection techs do it. They use a very small artists brush to layer VERY thin layers of paint IN the chip.
They keep doing this, letting it dry maybe a minute each layer, until the scratch is filled to the top, but not over filled. A quick rub with polishing compound and it is done and virtually invisible.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)