Why did Chartres Cathedral use heavily colored stained glass?

Artisans of the 12th and early 13th centuries, including those who worked on Chartres Cathedral, utilized a palette of deep, rich colors for their stained glass as a logical response to the architectural environment of the time.1

Because the light penetrating the interior of these early churches created a brilliance—and at times a harshness—in stark contrast to the surrounding darkness, the use of dense, saturated colors like blues and rubies was necessary to harmonize with the interior light levels. This approach allowed the windows to function effectively within the specific brightness scale established by the architecture of the period.1

Furthermore, the use of such glass was deeply rooted in the theological belief that light is the most direct manifestation of God. By filling the cathedral with these vibrant, colored windows, the artisans created an atmosphere that was intended to be both educational and spiritually evocative, transforming the interior space into a luminous, transcendent environment for pilgrims and worshippers.213