I got to see Classic Guns
http://www.classicgunsinc.com/ operation a couple of years ago. At the time, they had a commercial contract going. Virtually all the receivers were unique, but all had the same range of colors. As others have stated, it is matter of uniformity. New receivers they should all take the process pretty much the same. If every gun is exactly the same, then it most likely is not case coloring.
There is nothing secret about the process. It is widely known and taught. It is much more a matter of skill from doing hundreds of guns over the years. I will say that the work area can be rather dirty because of all the charcoal. Also, if the ovens were in operation, it would not have been safe to have people wandering around.
Pete
