I usually make my bluebird houses out of cedar fence boards an leave them unpainted, but I just made a Peterson bluebird house.

http://www.allwoodwork.com/plans/bluebird.htm

Because of the size of some of the lumbar, I made it out of pine boards, 2x4, 1x4 and 1x10. To preserve it I primed it and painted it with a flat white latex topcoat. I'm thinking white since I live in the south and it gets hot in the summer here. White should keep the box a little cooler, but does anyone know if there is a color that bluebirds would prefer?

I am also considering if I should paint it a light green so that it blends in with the lawn (actually bio-diversity, I don't have weeds) to make it harder to spot by owls. I am getting ready to put it on an existing pole that has a snake guard on it.