They infuse it with unicorn blood, known to change color near psycho-active substances.... Hahaha no, they "fume" gold or silver into the glass, which is pretty much vaporizing gold or silver into microscopic layers into the glass.... They layers only really come into effect when there is a black background (resin). Why that happens, not sure, probably has to do with the way black reflects light
Yup. Here's a decent breakdown I found on google; It's kind of like a "sheen" of color on the glass (cue Charlie Sheen jokes). The glass itself doesn't actually change color, but the black background of the resin makes the fuming on the glass show much more vibrantly. That's why many shops that sell fumed pieces will have a black mat that you can hold it up to in order to get an idea of what the "color change" might look like.