Why are the buildings in Greece white?

From: wikianswers.com

Houses are white washed because it keeps them cool. The houses themselves are very old and were built before modern conveniences like air-conditioning. The white wash reflects sunlight keeping them cool. They are even cool to the touch. The houses are white washed three times a year, with each person being responsible for white washing their own home. The blue domes represent the sea and sky.

From: uk.answers.yahoo.com

Houses in Greece are traditionally covered with a layer of plaster (Sovas in Greek). This layer is made out of calcium carbonate or lime stone. This plaster is regularly maintained by whitewashing by the same material. Now calcium carbonate is very bright white, so bright indeed that under the bright summer Greek sun can give you a headache. So people add a bit of blue color in the whitewash to "break" the brightness. So there is aways blue in the whitewash even if it looks white to you, and by adding more blue you can have nice white blue designs.

Why blue? and not green or yellow? Well actually there are green, yellow, red even purple. People were using what ever color they could find.
But blue was the most common because long time ago there was a cleaning agent called "loulaki" literally "lilac" that had that distinctive blue color. It had the texture of a toilet tablet and people using it for washing clothes and every house had some.

So you get your bucket of white wash, you drop one two tree whatever tablets of "loulaki" and away you go.

The village of Oios (Oia) on the island of Santorini, Greece

(geographic.org)