Pot metal
Pot metal is glass coloured all the way through. Metal oxides are added to the molten glass in the melting pot, which turns the whole batch one colour, and the glass is then blown or rolled into sheets. The name comes from this method and has nothing to do with the metal alloy of the same name.
Most medieval coloured glass was pot metal, and the colour came from the same few oxides that are still used today: copper for greens and blues, cobalt for deep blue, manganese for purple, and iron for greens and browns. Pot metal differs from flashed glass, in which a thin layer of colour sits on clear glass.
The depth of pot metal varies with thickness, which is why medieval blues glow where the glass is thick and look pale where it is thin.