Orange is the most prevalent color in fall, when the holiday occurs, and black is the color of death, which is why these are the most common colors for Halloween. The Celtics may have been the first to use this color combination for Halloween.
Besides being the color of autumn, orange signifies endurance and strength. Some people believe this strength and endurance was needed during the harvest season and the Celtics adopted the color to give them strength during battle.
Because Halloween was and, in some cultures, still is a way to celebrate the dead, and death is associated with an absence of light, black has become a color associated with death and, in turn, Halloween.