[johannes1234321]

Old buildings look so dark die to centries of dirt. Especially since industrialization and automobiles. But before that due to fires (for heating and light) For many of those old buildings there are light stones beneath the dark layer.

Same for glass windows, which in old times haven't been as clear as today possible, but still lighter than after centuries of dust settling.

[prewett]

The stone likely may have been painted, originally too. I asked a tour guide at Chartres about the why the floor was so rough and the walls so perfectly aligned. She said that the walls were not perfect, they painted over the stones and drew lines of perfect boundaries because the walls and ceiling represented heaven (perfect), while the floor represented earth (imperfect), so it used rough, variously-sized, unevenly fitted stones.

(Incidentally, this is related to why there are gargoyles on cathedrals. The church building is a microcosm of the cosmological universe. On the outside is untamed wild, which is where the monsters are (a monster being something we do not understand or cannot integrate, hence the abundance of chimeral combination of multiple animal and/or human parts). Inside the church is where the Kingdom is, with a baptistry at the beginning, because you enter the Kingdom through baptism. Then, the masses of the people in the Kingdom. And originally the altar was behind a screen, which represented the holy of holies, where God is; now this is often represented with a raised platform for the altar.)