The old glass - pre WW 1 - will turn color, mostly blue or purple hues
when exposed to bright sunshine.
Not all of it turns, but some will.
Glass makers used manganese until 1915
when they started using selenium,
both elements improved glass clarity.
Bottle diggers everywhere know and do this.
It just doesn't happen as much these
days because of antiquity laws.
(Means you can't dig up old dumps)