The most important aspect of stained glass is the glass itself. Glass is composed mainly of sand, silica. It is heated to a temperature of 2,000 degrees, until it is the consistency of taffy. The colors are caused by the addition of metallic oxides. The glass does not have a pigment in it like oil paints.
White light is a combination of all the colors of the rainbow.
To get green, you add iron to the sand mixture, causing the entire color spectrum to be absorbed by the glass, but green. To get pink color in glass, you must add pure gold to the mixture and so on. Because there is no pigments in glass, there is nothing to fade like oil paints. The color of glass will be as brilliant 700 years from now as it is today. Once you clean 700 years of contamination and build up of dirt and dust.