The rose window is a stone diagram of the cosmos, and the glass in it was never decoration but information. Blue for the sky, red for the martyrs, gold for the saints, green for the earth — and the whole thing had to be assembled from pieces small enough to be fired in a kiln and strong enough to hold up a roof.
Click any light in the window to change its glass.
Everything in a rose window is a piece of cut stone holding up everything else. The bar tracery — the thick straight mullions that divide the window into rectangular lights — was replaced in the thirteenth century by plate tracery, in which a single slab of stone is pierced with openings cut straight through, so that a petal can be a continuous curve rather than a rectangle.
That is what makes the window possible. A circle cannot be built out of rectangles, but it can be built out of holes. The masons cut the plate, the glaziers filled each hole with leaded glass, and the whole thing is held in a stone frame that carries the load.
Coloured glass is not transparent. It is coloured by adding metal oxides to the melt — copper for green and red, cobalt for blue, iron for yellow — and the colour comes from the metal itself absorbing the rest of the spectrum. Medieval glass is so saturated that it reads as a solid colour, which is why the windows look like jewels rather than pictures.
The glass was painted with vitreous enamel and fired again, and details like faces were added in grisaille — grey paint on white glass — because nobody has ever made a good red that will hold a fine line of black.