Traditionally black and orange is the colour of Halloween. Orange being the prevalent autumn tone, derived from the colours of the landscape, fallen leaves and of course pumpkins. It is also a colour which signifies strength and endurance.
Black depicts the colour of death and witches hats, cauldrons and black cats all signify the feast of All Hallows. And of course the blackness of the night cloaks the misdeeds and mayhem that may ensue later tonight!