This sounds pretty dumb, but I never noticed before that the Bat and Chalwyn red, yellow, and green UK warning lamps were painted to match the lens color. This makes sense as the lamps can then work in daylight as well as night. I'm sure it also made it easier for workmen to pull the correct color lamps from the back of a truck at night.
I notice the clear lens lamp is painted blue. I would have thought it would be white but it probably had some specialized use where the frame color was not so important.