I think you are on to something, and your answer will
likely be found in period catalogues. Much research
around those insulators has dramatically advanced our
understanding of them in the past 15 years, using the
old industry periodical publications and catalogues. The
internet has proven to be a real friend to research like
this.
What would be totally different about glass colors vs.
paint colors, is that a lot of care was given to paint color,
and keeping it consistent was important. With insulators,
glass batches varied with every scoop shovel of sand and
soda tossed into the furnace. And they most certainly did
not care about color .... at least not before 1930.
"Duty above all else except Honor"