Painting a porch ceiling light blue is thought to keep insects away from your home, with the cool tone repelling wasps and spiders, according to pest experts, as insects cannot register these shades and are less likely to be drawn towards it. However, this is not the most common or effective pest deterrent nor is it the main reason to paint their porch ceilings blue – especially in the southern states of America, which use blue-painted porch ceilings to ward off negative spirits instead.