Quoted from CaptainNeo:
i know from experience that backglasses have different colors depending on who printed them for certain games. Many had different companies making the same glass, much like they did for playfields. I can tell you first hand that there are at least 6 color variations for twilight zones. Depending when they were printed, and who printed them. Same with creature. So B/W must have had several companies making fields for them at that time. Same holds true for backglasses. Even with games like Robot. I've seen 3 different color schemes for the original backglasses. It happens, and none of them are wrong.
Forget different manufacturers. We're talking screen printing during the 70s. The chances of even the same manufacturer pouring the same color over a run are minimal. I have had nos Bobby Orr's with completely different skin colors and different blues. Even a small run, such as Embryon, has huge differences. To expect perfect matches is ridiculous. And remember, chances are you original bg has faded, uv damages colors so you're matching an incorrect color anyway.