It's high temp engine paint. I had a little Chrysler red laying around from my MOPAR project. Just mask off the bolt so only the top is exposed. Spray on the color lightly but enough to get into the depressions of the union jack. Let the paint tack up a little and then wipe the excess paint off the flat of the bolt with a rag that is barely damp with thinner or acetone. BTW might be worth mentioning that I painted mine prior to installing them!
