The luminosity of the sun.
“”The building is a building of light—it oscillates. People always say, ‘Well, what color is it meant to be?’ And I say, ‘It’s meant to be every color. It can be very dark on a broody day, and very bright on a sunny day.’ The material is supposed to react, and it does react, to the luminosity of the sun.”
David Adjaye in Dwell