hi Rosanna
in Nicole Anderson's "The Glory of Beads" there is a section describing how some innovative finishes were developed to add lustre or dye colour to the outside surface (like nail varnish) or just within the hole of drawn beads of transparent or translucent glass, which were basically painted on but not permanently fused to the surface.
Fine for fashion or theatrical costume embroidery but too perishable for anything that might need washing!
Accidentally the fumed-on iridescence or metallic coating which is applied to many czech beads at a temperature also may rub off, i've discovered.