Keith
It is factory applied paint I picked up a drop at a local sign supply place just to try it. Powder coated aluminum would produce similar results.
Polished aluminum would need to be coated with a topcoat called cermark or a similar product then laser etch the aluminum and wash off the remaining cermark. it would produce a dark grey to black mark according to the power / speed you engraved it at. Cermark is also used on other metals in order to etch them with a CO@ laser
You need to be careful with any polished or reflective surface as the laser bean can be reflected back at the lens and easily destroy it.