The lower half of each window was painted with a thin, translucent layer of yoghurt and varying amounts of blue pigment.
Since each window differed in hue, at one point during the day or night - for a moment only - a certain window would match the blue of the sky perfectly. The lower (painted) half and the upper (sky) half were then indistinguishable from each other.
The blues changed throughout the days and nights with the changing light and skies outside...and made blue skies even when the day was grey.
These photos taken at Duende Studios, Rotterdam, 2006.