The color pallet of medieval stained glass ranges from deep blues and reds to almost clear. The darker colors in particular draw attention to the holiness of the scenes since dark colors -- particularly purples, blues, and reds -- were difficult and expensive to create in arts (illuminations and cloth as well as stained glass) the Middle Ages, making them regal symbols.
Lighter colors were also useful for creating a feeling of opening the structure up to the heavens through the letting in of light. This gives visitors a feeling of light coming down from heaven into the space. Lighter-colored stained glass was used more and more beginning in the 13th century.