^ A civilization that old and long-lasting can't paint their dwellings, because each coating of paint is around 5 mils thick (0.005 inches) and you need to repaint every 5 to 10 years. Even in 200 years the paint would be 0.1 inches thick, causing severe door problems, but their civilization was much older and lasted for hundreds of thousands of years. In just 50,000 years each wall of a hallway would've moved in 2 and a half feet, forcing people to squeeze through sideways.
Thus, all sci-fi writers know that long-lasting, ancient civilizations couldn't have painted their walls.
That, or some producer told a set designer to just leave everything gray with a grime layer.