>https://www.answers.com/Q/Why_are_most_Greek_houses_painted_white_and_blue
white because it reflects the sunlight and thus the heat
blue because the blue roof indicates there is not a taxable enterprise inside the house. It initially indicated it was a church and the government wouldn't tax the operation if it was used as a church.
Then the businessmen wised up and painted their restaurants and every other enterprise with a blue roof. This is why Santorini appears to have three hundred churches even though it has only one priest.