I don't believe Colt certifies anyone past a warranty center but don't know that for sure.
Adams and Adams...has engraved any number of guns for both factories. And they are exceptionally nice gentlemen to deal on any engraving project. Which is hard to say about many engravers or engraving jobbers. In 25+ years and many projects with A&A I have never been disappointed.
ADAMS & ADAMS ENGRAVERS