We started a discussion about this with the guy at the paint store who swore that bees won't build their nests under a blue ceiling because they think they're too exposed. He said if you're going to do it, any color blue works (in the south, they use a blue-green color called "haint blue", Martha Stewart uses her own "Porch Ceiling Blue"). The best finish on an old ceiling is semi-gloss; high-gloss shows up every little imperfection, and flat won't reflect the light.
I heard it was a way to extend the sky over the porch.Have you ever noticed that The porch ceilings at Grandma's house are blue as well?!Food for thought.
(Taken from an Apartment Theropy article)