This has been debated before with no actual answer past our educated guesses. The above gun is a late, 2nd Generation Colt, NRA Comm from 1971 production. The trigger guard on this gun had turned purple by the time I bought it. I had the trigger guard reblued right after my purchase and it again turned purple.
I had an 3rd Gen production gun, 1st year production, (see photo below) that did exactly the same thing and when reblued, again turned purple.
This is a pair of the NRA guns from 1971 that are 1700 guns apart for serial number. The "purple" trigger guard is obvious (and really ugly) by comparison.
The "plum" colors were indicative of the earliest Kimber of Oregon rifles with cast receivers.
This is a photo of the back side of a Colt trigger guard that has changed to the "plum" or purple color.
A few years back I bought a couple of over size, rough cut, totally unfinished, but already serial numbered, Colt trigger guards off Ebay. This morning I spent some time fitting one of them to my "plum" colored NRA gun.
This is what I started with. By the serial number stamped on the trigger guard it is a Custom shop gun from 2004. Judge for yourself if Colt is using a cast trigger guard. I have no doubt in my mind the current trigger guards are an oversized casting that is then machined to fit, casting pits, mold marks and all.