Stained glass man here. Back in the day I used to make SG windows for churches - that died away. Glass is colored by dissolving metal salts into the melted glass like a dye in water - a small amount goes a long way. More here
https://www.bohaglass.co.uk/metals-used-to-create-coloured-g...
Classic stained glass uses a number of base transparent glasses, with the metal for color as the canvas, cutting into the various shapes for the art work painted onto and fired into this segmented canvas. They use lead 'cames' between them that are H shaped in cross section. The paints are various colored glasses with a high density of metal = dark color for viewing in thin sections. This glass is made of a lower melting glass we mixed in a small furnace and poured out on a flat. We then powdered this glass in a mortar and pestle and added 2-4% gum Arabic - a glue like binder. We also had black glass we powdered for lines. After powdering they looked like flour - the light scattering predominated. with a hint of the final color, black being the darkest. For use they were wetted and applied with small brushes to the drawing on the glass with fine black outlines. After drying, you could trim the edges with an exacto knife.
Then it was fired high enough to melt the powdered glass into a continuous phase, while the base glass softened and they bonded.The gum Arabic burned off into CO2. It was then cooled over 24-48 hours to anneal the glasses so their different contraction rates did not make the colored glass separate.
Then the final stained glass was assembled from the colored figure parts. Arms were segmented, clothing also, as large panels are at risk of fragmentation, and the base glass color also fitted, faces, hands. etc. The border was usually a geometric repeat, but was often legended. Then the puzzle was assembled with tin-lead solder and all gaps filled with cement based on red lead and linseed oil putty and carbon black. (now they use silicone seal). A few steel bars were added for support because the lead would gradually sag unless supported and it was fitted into the window.
That is classic stained glass. Pieces of colored glass in a pattern with no art or text is what we called a 'leaded light' Door panels, tiffany lamps are leaded lights.
It is an art that is near death, all the people I knew are gone, new churches are not built, old stained glass is scrapped due to lead environmental issues, and church attendance is under 5% in Canada.