The real trademarks are copyright (things like the Colt prancing horse logo, the words AR15, M16, M4 etc, and of course the text Colt Firearms Division, Colt Industries, Hartford, Conn, USA, next to the safety) so it would actually be illegal to have them engraved on without the permission of Colt, although it's doubtful Colt would come after you, especially since they would not know about it, although they have been known to be quite litigious in the past, they've sued a few other arms manufacturers for describing products as 'M4 type weapons' and even sued the US Army (and won) over who actually owned the M16 name. So finding a company prepared to break the law and engrave it might be tricky if the company is aware of how snotty Colt can be over their trademark. But, you could always simply do it yourself by knocking the trademarks up in Photoshop, printing a waterslide transfer off a laser printer, then fixing it with flat spray varnish. If you want assistance with that, I am an Adobe Certified Instructor in Photoshop, so I can knock it up for you.