Purple insulators.... made that way? Naturally turned by years of exposure to the sun? Or artificially nuked in a industrial food sterilizer or light box?
Some are made that way, top two rows are made for train signaling, so they used many different colors on purpose, none of these are nuked....you would be a damn fool to turn a clear H.G.C.O. signal into a purple one that is a fact. purple books at 500-600$ and clear books for 1250-1500$ Purples were more than likely clear when made at the factory but as you stated, 100 years of sun rays mixed with manganese glass turns.
Thanks, I'm sure there are guys nuking them just like they are with bottles these days, you also have guys nuking cobalt Bromos turning them an odd green color and selling them for good money on Ebay...