If you have, or have access to, a high intensity flashlight (xenon bulb or white LED), you could place the flashlight on one side of the tail light. The light beam will be strong enough for you to see what is the color of the coating in the bulb itself.
I was reading some material which had to do with various coatings used in light bulbs and radio tubes. Since this light was intended to be viewed in darkened conditions, the coating inside the tail light is probably a reflective finish that intensifies the the illumination of the bulb, similar to the coating used in old vacuum tubes in radios and televisions.
I'll pipe in and say that the color is greenish. Then again, I don't know what I'm talking about 37.97453% of the time.