1) Clean the area well. I used brake cleaner with a wire hand brush.
(Warning brake cleaner will damage most finishes!)

2) With a Q-tip (Not one you just picked the carrots out of your ears with.) apply some white paint.

3) After the white paint dries. Apply a dark color paint. (Try not to cover the stamped code completely)
     Let the darker paint set, but not dry completely.
 

4) With a toothpick gently remove the darker paint from the stamped code.
     ( You might find it easier if you cut the toothpick in half and use the fat end, which rides on top of the stamped code edges.)