Bottle Trees were sometimes placed near the entrance to the house, so the spirits did not enter the house with you. The spirits would become mesmerized by the play of the sunlight through the colors of the glass, and become trapped inside the bottles.
in gullah areas of the south, the color blue is most significant. it is the color to ward off evil spirits. many houses have the window and door trims painted this bright blue. most bottle trees will have the blue bottles.